pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...

toxin A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849– ...

illness A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that ar ...

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky's 1 ...

immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splinte ...

Mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

innate {{Short pages monitor Among the many varieties of

microorganisms A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

, relatively few cause disease in otherwise healthy individuals.This section incorporate
public domain
materials included in the text
Medical Microbiology
Fourth Edition
Chapter 8
(1996). Baron, Samuel MD. The University of Texas Medical Branch at Galveston. Infectious disease results from the interplay between those few

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

s and the defenses of the hosts they infect. The appearance and severity of disease resulting from any pathogen depend upon the ability of that pathogen to damage the host as well as the ability of the host to resist the pathogen. However, a host's immune system can also cause damage to the host itself in an attempt to control the infection. Clinicians, therefore, classify infectious microorganisms or microbes according to the status of host defenses – either as ''primary pathogens'' or as ''

opportunistic pathogens An opportunistic infection is an infection caused by pathogens (bacteria, fungi, parasites or viruses) that take advantage of an opportunity not normally available. These opportunities can stem from a variety of sources, such as a weakened immune ...

''.

= Primary pathogens

= Primary pathogens cause disease as a result of their presence or activity within the normal, healthy host, and their intrinsic

virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ca ...

(the severity of the disease they cause) is, in part, a necessary consequence of their need to reproduce and spread. Many of the most common primary pathogens of humans only infect humans, however, many serious diseases are caused by organisms acquired from the environment or that infect non-human hosts.

= Opportunistic pathogens

= Opportunistic pathogens can cause an infectious disease in a host with depressed resistance (

immunodeficiency Immunodeficiency, also known as immunocompromisation, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that a ...

) or if they have unusual access to the inside of the body (for example, via

trauma Trauma most often refers to: * Major trauma, in physical medicine, severe physical injury caused by an external source * Psychological trauma, a type of damage to the psyche that occurs as a result of a severely distressing event *Traumatic i ...

).

Opportunistic infection An opportunistic infection is an infection caused by pathogens (bacteria, fungi, parasites or viruses) that take advantage of an opportunity not normally available. These opportunities can stem from a variety of sources, such as a weakened immune ...

may be caused by microbes ordinarily in contact with the host, such as

pathogenic bacteria Pathogenic bacteria are bacteria that can cause disease. This article focuses on the bacteria that are pathogenic to humans. Most species of bacteria are harmless and are often Probiotic, beneficial but others can cause infectious diseases. The n ...

or fungi in the

gastrointestinal The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...

or the

upper respiratory tract The respiratory tract is the subdivision of the respiratory system involved with the process of respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa. Air is breathed in through the nose to th ...

, and they may also result from (otherwise innocuous) microbes acquired from other hosts (as in '' Clostridium difficile''

colitis Colitis is swelling or inflammation of the large intestine ( colon). Colitis may be acute and self-limited or long-term. It broadly fits into the category of digestive diseases. In a medical context, the label ''colitis'' (without qualification) ...

) or from the environment as a result of traumatic introduction (as in

surgical Surgery ''cheirourgikē'' (composed of χείρ, "hand", and ἔργον, "work"), via la, chirurgiae, meaning "hand work". is a medical specialty that uses operative manual and instrumental techniques on a person to investigate or treat a pat ...

wound infections or

compound fracture A bone fracture (abbreviated FRX or Fx, Fx, or #) is a medical condition in which there is a partial or complete break in the continuity of any bone in the body. In more severe cases, the bone may be broken into several fragments, known as a '' ...

s). An opportunistic disease requires impairment of host defenses, which may occur as a result of genetic defects (such as

chronic granulomatous disease Chronic granulomatous disease (CGD), also known as Bridges–Good syndrome, chronic granulomatous disorder, and Quie syndrome, is a diverse group of hereditary diseases in which certain cells of the immune system have difficulty forming the reacti ...

), exposure to

antimicrobial An antimicrobial is an agent that kills microorganisms or stops their growth. Antimicrobial medicines can be grouped according to the microorganisms they act primarily against. For example, antibiotics are used against bacteria, and antifungals ar ...

drugs or

immunosuppressive Immunosuppression is a reduction of the activation or efficacy of the immune system. Some portions of the immune system itself have immunosuppressive effects on other parts of the immune system, and immunosuppression may occur as an adverse reacti ...

chemicals (as might occur following

poison Poison is a chemical substance that has a detrimental effect to life. The term is used in a wide range of scientific fields and industries, where it is often specifically defined. It may also be applied colloquially or figuratively, with a broa ...

ing or

cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...

chemotherapy Chemotherapy (often abbreviated to chemo and sometimes CTX or CTx) is a type of cancer treatment that uses one or more anti-cancer drugs (chemotherapeutic agents or alkylating agents) as part of a standardized chemotherapy regimen. Chemotherap ...

), exposure to

ionizing radiation Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel ...

, or as a result of an infectious disease with immunosuppressive activity (such as with

measles Measles is a highly contagious infectious disease caused by measles virus. Symptoms usually develop 10–12 days after exposure to an infected person and last 7–10 days. Initial symptoms typically include fever, often greater than , cough, ...

,

malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. S ...

or HIV disease). Primary pathogens may also cause more severe disease in a host with depressed resistance than would normally occur in an immunosufficient host.

= Secondary infection

= While a primary infection can practically be viewed as the

root cause Root cause may refer to: * "Root Cause" (''Person of Interest''), a TV show episode * Root cause analysis In science Science is a systematic endeavor that builds and organizes knowledge in the form of testable explanations and predic ...

of an individual's current health problem, a secondary infection is a

sequela A sequela (, ; usually used in the plural, sequelae ) is a pathological condition resulting from a disease, injury, therapy, or other trauma. Derived from the Latin word, meaning “sequel”, it is used in the medical field to mean a complication ...

or complication of that root cause. For example, an infection due to a

burn A burn is an injury to skin, or other tissues, caused by heat, cold, electricity, chemicals, friction, or ultraviolet radiation (like sunburn). Most burns are due to heat from hot liquids (called scalding), solids, or fire. Burns occur mainl ...

or

penetrating trauma Penetrating trauma is an open wound injury that occurs when an object pierces the skin and enters a tissue of the body, creating a deep but relatively narrow entry wound. In contrast, a blunt or ''non-penetrating'' trauma may have some deep dama ...

(the root cause) is a secondary infection. Primary pathogens often cause primary infection and often cause secondary infection. Usually, opportunistic infections are viewed as secondary infections (because immunodeficiency or

injury An injury is any physiological damage to living tissue caused by immediate physical stress. An injury can occur intentionally or unintentionally and may be caused by blunt trauma, penetrating trauma, burning, toxic exposure, asphyxiation, o ...

was the predisposing factor).

= Other types of infection

= Other types of infection consist of mixed, iatrogenic, nosocomial, and community-acquired infection. A mixed infection is an infection that is caused by two or more pathogens. An example of this is

appendicitis Appendicitis is inflammation of the appendix. Symptoms commonly include right lower abdominal pain, nausea, vomiting, and decreased appetite. However, approximately 40% of people do not have these typical symptoms. Severe complications of a rup ...

, which is caused by ''Bacteroides fragilis'' and ''Escherichia coli''. The second is an iatrogenic infection. This type of infection is one that is transmitted from a health care worker to a patient. A nosocomial infection is also one that occurs in a health care setting. Nosocomial infections are those that are acquired during a hospital stay. Lastly, a community-acquired infection is one in which the infection is acquired from a whole community.

Infectious or not

One manner of proving that a given disease is infectious, is to satisfy

Koch's postulates Koch's postulates ( )"Koch"
''

Robert Koch Heinrich Hermann Robert Koch ( , ; 11 December 1843 – 27 May 1910) was a German physician and microbiologist. As the discoverer of the specific causative agents of deadly infectious diseases including tuberculosis, cholera (though the Vibrio ...

), which require that first, the

infectious agent In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

be identifiable only in patients who have the disease, and not in healthy controls, and second, that patients who contract the infectious agent also develop the disease. These postulates were first used in the discovery that

Mycobacteria ''Mycobacterium'' is a genus of over 190 species in the phylum Actinomycetota, assigned its own family, Mycobacteriaceae. This genus includes pathogens known to cause serious diseases in mammals, including tuberculosis ('' M. tuberculosis'') and ...

species cause

tuberculosis Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, in ...

. However, Koch's postulates cannot usually be tested in modern practice for ethical reasons. Proving them would require experimental infection of a healthy individual with a

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

produced as a pure culture. Conversely, even clearly infectious diseases do not always meet the infectious criteria; for example, ''

Treponema pallidum ''Treponema pallidum'', formerly known as ''Spirochaeta pallida'', is a spirochaete bacterium with various subspecies that cause the diseases syphilis, bejel (also known as endemic syphilis), and yaws. It is transmitted only among humans. It is ...

'', the causative

spirochete A spirochaete () or spirochete is a member of the phylum Spirochaetota (), (synonym Spirochaetes) which contains distinctive diderm (double-membrane) gram-negative bacteria, most of which have long, helically coiled (corkscrew-shaped or s ...

of

syphilis Syphilis () is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms of syphilis vary depending in which of the four stages it presents (primary, secondary, latent, an ...

, cannot be cultured ''in vitro'' – however the organism can be cultured in rabbit

testes A testicle or testis (plural testes) is the male reproductive gland or gonad in all bilaterians, including humans. It is homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, primarily testoster ...

. It is less clear that a pure culture comes from an animal source serving as host than it is when derived from microbes derived from plate culture.

Epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and shapes policy decisions and evidenc ...

, or the study and analysis of who, why and where disease occurs, and what determines whether various populations have a disease, is another important tool used to understand infectious disease. Epidemiologists may determine differences among groups within a population, such as whether certain age groups have a greater or lesser rate of infection; whether groups living in different neighborhoods are more likely to be infected; and by other factors, such as gender and race. Researchers also may assess whether a disease

outbreak In epidemiology, an outbreak is a sudden increase in occurrences of a disease when cases are in excess of normal expectancy for the location or season. It may affect a small and localized group or impact upon thousands of people across an entire ...

is sporadic, or just an occasional occurrence;

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found elsew ...

, with a steady level of regular cases occurring in a region;

epidemic An epidemic (from Ancient Greek, Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of patients among a given population within an area in a short period of time. Epidemics ...

, with a fast arising, and unusually high number of cases in a region; or

pandemic A pandemic () is an epidemic of an infectious disease that has spread across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. A widespread endemic (epidemiology), endemic disease wi ...

, which is a global epidemic. If the cause of the infectious disease is unknown, epidemiology can be used to assist with tracking down the sources of infection.

Contagiousness

Infectious diseases are sometimes called contagious diseases when they are easily transmitted by contact with an ill person or their secretions (e.g.,

influenza Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms ...

). Thus, a contagious disease is a subset of infectious disease that is especially infective or easily transmitted. Other types of infectious, transmissible, or communicable diseases with more specialized routes of infection, such as vector transmission or sexual transmission, are usually not regarded as "contagious", and often do not require medical isolation (sometimes loosely called

quarantine A quarantine is a restriction on the movement of people, animals and goods which is intended to prevent the spread of disease or pests. It is often used in connection to disease and illness, preventing the movement of those who may have been ...

) of those affected. However, this specialized connotation of the word "contagious" and "contagious disease" (easy transmissibility) is not always respected in popular use. Infectious diseases are commonly transmitted from person to person through direct contact. The types of contact are through person to person and droplet spread. Indirect contact such as airborne transmission, contaminated objects, food and drinking water, animal person contact, animal reservoirs, insect bites, and environmental reservoirs are another way infectious diseases are transmitted.

By anatomic location

Infections can be classified by the anatomic location or

organ system An organ system is a biological system consisting of a group of organs that work together to perform one or more functions. Each organ has a specialized role in a plant or animal body, and is made up of distinct tissues. Plants Plants have ...

infected, including: *

Urinary tract infection A urinary tract infection (UTI) is an infection that affects part of the urinary tract. When it affects the lower urinary tract it is known as a bladder infection (cystitis) and when it affects the upper urinary tract it is known as a kidney ...

*

Skin infection A skin infection is an infection of the skin in humans and other animals, that can also affect the associated soft tissues such as loose connective tissue and mucous membranes. They comprise a category of infections termed skin and skin structure ...

*

Respiratory tract infection Respiratory tract infections (RTIs) are infectious diseases involving the respiratory tract. An infection of this type usually is further classified as an upper respiratory tract infection (URI or URTI) or a lower respiratory tract infection (LRI ...

*

Odontogenic infection An odontogenic infection is an infection that originates within a tooth or in the closely surrounding tissues. The term is derived from '' odonto-'' (Ancient Greek: , – 'tooth') and '' -genic'' (Ancient Greek: , ; – 'birth'). The most common ...

(an infection that originates within a

tooth A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...

or in the closely surrounding tissues) *

Vaginal infections Vaginitis, also known as vulvovaginitis, is inflammation of the vagina and vulva. Symptoms may include itching, burning, pain, discharge, and a bad smell. Certain types of vaginitis may result in complications during pregnancy. The three main ...

*

Intra-amniotic infection Chorioamnionitis, also known as intra-amniotic infection (IAI), is inflammation of the fetal membranes ( amnion and chorion), usually due to bacterial infection. In 2015, a National Institute of Child Health and Human Development Workshop expert ...

In addition, locations of

inflammation Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...

where infection is the most common cause include

pneumonia Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severity ...

,

meningitis Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, headache, and neck stiffness. Other symptoms include confusion or ...

and

salpingitis Salpingitis is an infection causing inflammation in the Fallopian tubes (also called ''salpinges''). It is often included in the umbrella term of pelvic inflammatory disease (PID), along with endometritis, oophoritis, myometritis, parametritis, ...

.

Prevention

Techniques like hand washing, wearing gowns, and wearing face masks can help prevent infections from being passed from one person to another.

Aseptic technique Asepsis is the state of being free from disease-causing micro-organisms (such as pathogenic bacteria, viruses, pathogenic fungi, and parasites). There are two categories of asepsis: medical and surgical. The modern day notion of asepsis is deriv ...

was introduced in medicine and surgery in the late 19th century and greatly reduced the incidence of infections caused by surgery. Frequent

hand washing Hand washing (or handwashing), also known as hand hygiene, is the act of cleaning one's hands with soap, soap or handwash and water to remove viruses/bacteria/microorganisms, dirt, grease, or other harmful and unwanted substances stuck to the ...

remains the most important defense against the spread of unwanted organisms. There are other forms of prevention such as avoiding the use of illicit drugs, using a

condom A condom is a sheath-shaped barrier device used during sexual intercourse to reduce the probability of pregnancy or a sexually transmitted infection (STI). There are both male and female condoms. With proper use—and use at every act of in ...

, wearing gloves, and having a healthy lifestyle with a balanced diet and regular exercise. Cooking foods well and avoiding foods that have been left outside for a long time is also important. Antimicrobial substances used to prevent transmission of infections include: *

antiseptics An antiseptic (from Greek ἀντί ''anti'', "against" and σηπτικός ''sēptikos'', "putrefactive") is an antimicrobial substance or compound that is applied to living tissue/skin to reduce the possibility of infection, sepsis, or putre ...

, which are applied to living tissue/

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other cuticle, animal coverings, such as the arthropod exoskeleton, have diffe ...

*

disinfectant A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than st ...

s, which destroy microorganisms found on non-living objects. *

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of ...

s, called

prophylactic Preventive healthcare, or prophylaxis, consists of measures taken for the purposes of disease prevention.Hugh R. Leavell and E. Gurney Clark as "the science and art of preventing disease, prolonging life, and promoting physical and mental hea ...

when given as prevention rather as treatment of infection. However, long term use of antibiotics leads to resistance of bacteria. While humans do not become immune to antibiotics, the bacteria does. Thus, avoiding using antibiotics longer than necessary helps preventing bacteria from forming mutations that aide in antibiotic resistance. One of the ways to prevent or slow down the transmission of infectious diseases is to recognize the different characteristics of various diseases. Some critical disease characteristics that should be evaluated include

virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ca ...

, distance traveled by those affected, and level of contagiousness. The human strains of

Ebola Ebola, also known as Ebola virus disease (EVD) and Ebola hemorrhagic fever (EHF), is a viral hemorrhagic fever in humans and other primates, caused by ebolaviruses. Symptoms typically start anywhere between two days and three weeks after becom ...

virus, for example, incapacitate those infected extremely quickly and kill them soon after. As a result, those affected by this disease do not have the opportunity to travel very far from the initial infection zone. Also, this virus must spread through skin lesions or permeable membranes such as the eye. Thus, the initial stage of

Ebola Ebola, also known as Ebola virus disease (EVD) and Ebola hemorrhagic fever (EHF), is a viral hemorrhagic fever in humans and other primates, caused by ebolaviruses. Symptoms typically start anywhere between two days and three weeks after becom ...

is not very contagious since its victims experience only internal hemorrhaging. As a result of the above features, the spread of Ebola is very rapid and usually stays within a relatively confined geographical area. In contrast, the

human immunodeficiency virus The human immunodeficiency viruses (HIV) are two species of ''Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause AIDS, acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the ...

(

HIV The human immunodeficiency viruses (HIV) are two species of ''Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immune ...

) kills its victims very slowly by attacking their immune system. As a result, many of its victims transmit the virus to other individuals before even realizing that they are carrying the disease. Also, the relatively low virulence allows its victims to travel long distances, increasing the likelihood of an

epidemic An epidemic (from Ancient Greek, Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of patients among a given population within an area in a short period of time. Epidemics ...

. Another effective way to decrease the transmission rate of infectious diseases is to recognize the effects of

small-world networks A small-world network is a type of mathematical graph in which most nodes are not neighbors of one another, but the neighbors of any given node are likely to be neighbors of each other and most nodes can be reached from every other node by a sm ...

. In epidemics, there are often extensive interactions within hubs or groups of infected individuals and other interactions within discrete hubs of susceptible individuals. Despite the low interaction between discrete hubs, the disease can jump and spread in a susceptible hub via a single or few interactions with an infected hub. Thus, infection rates in small-world networks can be reduced somewhat if interactions between individuals within infected hubs are eliminated (Figure 1). However, infection rates can be drastically reduced if the main focus is on the prevention of transmission jumps between hubs. The use of needle exchange programs in areas with a high density of drug users with HIV is an example of the successful implementation of this treatment method. Another example is the use of ring culling or vaccination of potentially susceptible livestock in adjacent farms to prevent the spread of the

foot-and-mouth Foot-and-mouth disease (FMD) or hoof-and-mouth disease (HMD) is an infectious and sometimes fatal viral disease that affects cloven-hoofed animals, including domestic and wild bovids. The virus causes a high fever lasting two to six days, follow ...

virus in 2001. A general method to prevent transmission of

vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...

-borne pathogens is

pest control Pest control is the regulation or management of a species defined as a pest; any animal, plant or fungus that impacts adversely on human activities or environment. The human response depends on the importance of the damage done and will range ...

. In cases where infection is merely suspected, individuals may be

quarantined A quarantine is a restriction on the movement of people, animals and goods which is intended to prevent the spread of disease or pests. It is often used in connection to disease and illness, preventing the movement of those who may have been ...

until the incubation period has passed and the disease manifests itself or the person remains healthy. Groups may undergo quarantine, or in the case of communities, a

cordon sanitaire ''Cordon sanitaire'' () is French for "sanitary cordon". It may refer to: *Cordon sanitaire (medicine), a cordon that quarantines an area during an infectious disease outbreak *Cordon sanitaire (politics), refusal to cooperate with certain politic ...

may be imposed to prevent infection from spreading beyond the community, or in the case of

protective sequestration Protective sequestration, in public health, is social distancing measures taken to protect a small, defined, and still-healthy population from outsiders during an epidemic (or pandemic) ''before'' the infection reaches that population. It is someti ...

, into a community. Public health authorities may implement other forms of

social distancing In public health, social distancing, also called physical distancing, (NB. Regula Venske is president of the PEN Centre Germany.) is a set of non-pharmaceutical interventions or measures intended to prevent the spread of a contagious disea ...

, such as school closings, to control an epidemic.

Immunity

Infection with most pathogens does not result in death of the host and the offending organism is ultimately cleared after the symptoms of the disease have waned. This process requires immune mechanisms to kill or inactivate the

inoculum In biology, inoculum refers to the source material used for inoculation. ''Inoculum'' may refer to: * In medicine, material that is the source of the inoculation in a vaccine * In microbiology, propagules: cells, tissue, or viruses that are used t ...

of the pathogen. Specific acquired

immunity Immunity may refer to: Medicine * Immunity (medical), resistance of an organism to infection or disease * ''Immunity'' (journal), a scientific journal published by Cell Press Biology * Immune system Engineering * Radiofrequence immunity desc ...

against infectious diseases may be mediated by

antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...

and/or

T lymphocyte A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell rec ...

s. Immunity mediated by these two factors may be manifested by: * a direct effect upon a pathogen, such as antibody-initiated

complement A complement is something that completes something else. Complement may refer specifically to: The arts * Complement (music), an interval that, when added to another, spans an octave ** Aggregate complementation, the separation of pitch-class ...

-dependent bacteriolysis, opsonoization,

phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...

and killing, as occurs for some bacteria, * neutralization of viruses so that these organisms cannot enter cells, * or by T lymphocytes, which will kill a cell parasitized by a microorganism. The immune system response to a microorganism often causes symptoms such as a high

fever Fever, also referred to as pyrexia, is defined as having a body temperature, temperature above the human body temperature, normal range due to an increase in the body's temperature Human body temperature#Fever, set point. There is not a single ...

and

inflammation Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...

, and has the potential to be more devastating than direct damage caused by a microbe. Resistance to infection (

immunity Immunity may refer to: Medicine * Immunity (medical), resistance of an organism to infection or disease * ''Immunity'' (journal), a scientific journal published by Cell Press Biology * Immune system Engineering * Radiofrequence immunity desc ...

) may be acquired following a disease, by asymptomatic carriage of the pathogen, by harboring an organism with a similar structure (crossreacting), or by

vaccination Vaccination is the administration of a vaccine to help the immune system develop immunity from a disease. Vaccines contain a microorganism or virus in a weakened, live or killed state, or proteins or toxins from the organism. In stimulating ...

. Knowledge of the protective antigens and specific acquired host immune factors is more complete for primary pathogens than for

opportunistic pathogen An opportunistic infection is an infection caused by pathogens (bacteria, fungi, parasites or viruses) that take advantage of an opportunity not normally available. These opportunities can stem from a variety of sources, such as a weakened immune ...

s. There is also the phenomenon of

herd immunity Herd immunity (also called herd effect, community immunity, population immunity, or mass immunity) is a form of indirect protection that applies only to contagious diseases. It occurs when a sufficient percentage of a population has become imm ...

which offers a measure of protection to those otherwise vulnerable people when a large enough proportion of the population has acquired immunity from certain infections. Immune resistance to an infectious disease requires a critical level of either antigen-specific antibodies and/or T cells when the host encounters the pathogen. Some individuals develop natural serum antibodies to the surface

polysaccharide Polysaccharides (), or polycarbohydrates, are the most abundant carbohydrates found in food. They are long chain polymeric carbohydrates composed of monosaccharide units bound together by glycosidic linkages. This carbohydrate can react with wa ...

s of some agents although they have had little or no contact with the agent, these natural antibodies confer specific protection to adults and are passively transmitted to newborns.

Host genetic factors

The organism that is the target of an infecting action of a specific infectious agent is called the host. The host harbouring an agent that is in a mature or sexually active stage phase is called the definitive host. The intermediate host comes in contact during the larvae stage. A host can be anything living and can attain to asexual and sexual reproduction. The clearance of the pathogens, either treatment-induced or spontaneous, it can be influenced by the genetic variants carried by the individual patients. For instance, for genotype 1 hepatitis C treated with

Pegylated interferon-alpha-2a Pegylated interferon alfa-2a, sold under the brand name Pegasys among others, is medication used to treat hepatitis C and hepatitis B. For hepatitis C it is typically used together with ribavirin and cure rates are between 24 and 92%. For hepati ...

or

Pegylated interferon-alpha-2b Pegylated interferon alfa-2b is a drug used to treat melanoma, as an adjuvant therapy to surgery. Also used to treat hepatitis C (typically, in combination with ribavarin), it is no longer recommended due to poor efficacy and adverse side-effects ...

(brand names Pegasys or PEG-Intron) combined with

ribavirin Ribavirin, also known as tribavirin, is an antiviral medication used to treat RSV infection, hepatitis C and some viral hemorrhagic fevers. For hepatitis C, it is used in combination with other medications such as simeprevir, sofosbuvir, pegin ...

, it has been shown that genetic polymorphisms near the human IL28B gene, encoding interferon lambda 3, are associated with significant differences in the treatment-induced clearance of the virus. This finding, originally reported in Nature, showed that genotype 1 hepatitis C patients carrying certain genetic variant alleles near the IL28B gene are more possibly to achieve sustained virological response after the treatment than others. Later report from Nature demonstrated that the same genetic variants are also associated with the natural clearance of the genotype 1 hepatitis C virus.

Treatments

When infection attacks the body, ''anti-infective'' drugs can suppress the infection. Several broad types of anti-infective drugs exist, depending on the type of organism targeted; they include antibacterial (

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of ...

; including antitubercular),

antiviral Antiviral drugs are a class of medication used for treating viral infections. Most antivirals target specific viruses, while a broad-spectrum antiviral is effective against a wide range of viruses. Unlike most antibiotics, antiviral drugs do no ...

,

antifungal An antifungal medication, also known as an antimycotic medication, is a pharmaceutical fungicide or fungistatic used to treat and prevent mycosis such as athlete's foot, ringworm, candidiasis (thrush), serious systemic infections such as crypto ...

and

antiparasitic Antiparasitics are a class of medications which are indicated for the treatment of parasitic diseases, such as those caused by helminths, amoeba, ectoparasites, parasitic fungi, and protozoa, among others. Antiparasitics target the parasitic agent ...

(including

antiprotozoal Antiprotozoal agents ( ATC code: ATC P01) is a class of pharmaceuticals used in treatment of protozoan infection. A paraphyletic group, protozoans have little in common with each other. For example, ''Entamoeba histolytica'', a unikont eukaryotic ...

and

antihelminthic Anthelmintics or antihelminthics are a group of antiparasitic drugs that expel parasitic worms (helminths) and other internal parasites from the body by either stunning or killing them and without causing significant damage to the host. They may a ...

) agents. Depending on the severity and the type of infection, the antibiotic may be given by mouth or by injection, or may be applied

topical A topical medication is a medication that is applied to a particular place on or in the body. Most often topical medication means application to body surfaces such as the skin or mucous membranes to treat ailments via a large range of classes ...

ly. Severe infections of the

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

are usually treated with

intravenous Intravenous therapy (abbreviated as IV therapy) is a medical technique that administers fluids, medications and nutrients directly into a person's vein. The intravenous route of administration is commonly used for rehydration or to provide nutrie ...

antibiotics. Sometimes, multiple antibiotics are used in case there is resistance to one antibiotic. Antibiotics only work for bacteria and do not affect viruses. Antibiotics work by slowing down the multiplication of bacteria or killing the bacteria. The most common classes of antibiotics used in medicine include

penicillin Penicillins (P, PCN or PEN) are a group of β-lactam antibiotics originally obtained from ''Penicillium'' moulds, principally '' P. chrysogenum'' and '' P. rubens''. Most penicillins in clinical use are synthesised by P. chrysogenum using ...

,

cephalosporin The cephalosporins (sg. ) are a class of β-lactam antibiotics originally derived from the fungus ''Acremonium'', which was previously known as ''Cephalosporium''. Together with cephamycins, they constitute a subgroup of β-lactam antibiotics ...

s,

aminoglycoside Aminoglycoside is a medicinal and bacteriologic category of traditional Gram-negative antibacterial medications that inhibit protein synthesis and contain as a portion of the molecule an amino-modified glycoside (sugar). The term can also refer ...

s,

macrolide The Macrolides are a class of natural products that consist of a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. The lactone rings are usually 14-, 15-, or 16-membered. Macrol ...

s,

quinolones Quinolone may refer to: * 2-Quinolone * 4-Quinolone 4-Quinolone is an organic compound derived from quinoline. It and 2-quinolone are the two most important parent (meaning simplified) quinolones. 4-Quinolone exists in equilibrium with a mino ...

and

tetracyclines Tetracyclines are a group of broad-spectrum antibiotic compounds that have a common basic structure and are either isolated directly from several species of ''Streptomyces'' bacteria or produced semi-synthetically from those isolated compounds. T ...

. Not all infections require treatment, and for many self-limiting infections the treatment may cause more side-effects than benefits.

Antimicrobial stewardship Antimicrobial stewardship is the systematic effort to educate and persuade prescribers of antimicrobials to follow evidence-based prescribing, in order to stem antimicrobial overuse, and thus antimicrobial resistance. AMS has been an organized effo ...

is the concept that healthcare providers should treat an infection with an antimicrobial that specifically works well for the target pathogen for the shortest amount of time and to only treat when there is a known or highly suspected pathogen that will respond to the medication.

Epidemiology

Infectious and parasitic diseases world map-Deaths per million persons-WHO2012

Infectious and parasitic diseases world map - DALY - WHO2004

In 2010, about 10 million people died of infectious diseases. The

World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level of h ...

collects information on global deaths by International Classification of Disease (ICD) code categories. The following table lists the top infectious disease by number of deaths in 2002. 1993 data is included for comparison. The top three single agent/disease killers are

HIV The human immunodeficiency viruses (HIV) are two species of ''Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immune ...

/

AIDS Human immunodeficiency virus infection and acquired immunodeficiency syndrome (HIV/AIDS) is a spectrum of conditions caused by infection with the human immunodeficiency virus (HIV), a retrovirus. Following initial infection an individual m ...

, TB and

malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. S ...

. While the number of deaths due to nearly every disease have decreased, deaths due to HIV/AIDS have increased fourfold. Childhood diseases include

pertussis Whooping cough, also known as pertussis or the 100-day cough, is a highly contagious bacterial disease. Initial symptoms are usually similar to those of the common cold with a runny nose, fever, and mild cough, but these are followed by two or ...

,

poliomyelitis Poliomyelitis, commonly shortened to polio, is an infectious disease caused by the poliovirus. Approximately 70% of cases are asymptomatic; mild symptoms which can occur include sore throat and fever; in a proportion of cases more severe sym ...

,

diphtheria Diphtheria is an infection caused by the bacterium '' Corynebacterium diphtheriae''. Most infections are asymptomatic or have a mild clinical course, but in some outbreaks more than 10% of those diagnosed with the disease may die. Signs and s ...

,

measles Measles is a highly contagious infectious disease caused by measles virus. Symptoms usually develop 10–12 days after exposure to an infected person and last 7–10 days. Initial symptoms typically include fever, often greater than , cough, ...

and

tetanus Tetanus, also known as lockjaw, is a bacterial infection caused by ''Clostridium tetani'', and is characterized by muscle spasms. In the most common type, the spasms begin in the jaw and then progress to the rest of the body. Each spasm usually ...

. Children also make up a large percentage of lower respiratory and diarrheal deaths. In 2012, approximately 3.1 million people have died due to lower respiratory infections, making it the number 4 leading cause of death in the world.

Historic pandemics

With their potential for unpredictable and explosive impacts, infectious diseases have been major actors in human

history History (derived ) is the systematic study and the documentation of the human activity. The time period of event before the History of writing#Inventions of writing, invention of writing systems is considered prehistory. "History" is an umbr ...

. A

pandemic A pandemic () is an epidemic of an infectious disease that has spread across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. A widespread endemic (epidemiology), endemic disease wi ...

(or global

epidemic An epidemic (from Ancient Greek, Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of patients among a given population within an area in a short period of time. Epidemics ...

) is a disease that affects people over an extensive geographical area. For example: *

Plague of Justinian The plague of Justinian or Justinianic plague (541–549 AD) was the first recorded major outbreak of the first plague pandemic, the first Old World pandemic of plague, the contagious disease caused by the bacterium ''Yersinia pestis''. The dis ...

, from 541 to 542, killed between 50% and 60% of Europe's population. * The

Black Death The Black Death (also known as the Pestilence, the Great Mortality or the Plague) was a bubonic plague pandemic occurring in Western Eurasia and North Africa from 1346 to 1353. It is the most fatal pandemic recorded in human history, causi ...

of 1347 to 1352 killed 25 million in Europe over 5 years. The plague reduced the old world population from an estimated 450 million to between 350 and 375 million in the 14th century. * The introduction of

smallpox Smallpox was an infectious disease caused by variola virus (often called smallpox virus) which belongs to the genus Orthopoxvirus. The last naturally occurring case was diagnosed in October 1977, and the World Health Organization (WHO) c ...

, measles, and

typhus Typhus, also known as typhus fever, is a group of infectious diseases that include epidemic typhus, scrub typhus, and murine typhus. Common symptoms include fever, headache, and a rash. Typically these begin one to two weeks after exposure. ...

to the areas of Central and South America by European explorers during the 15th and 16th centuries caused pandemics among the native inhabitants. Between 1518 and 1568 disease pandemics are said to have caused the population of

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

to fall from 20 million to 3 million. * The first European

influenza Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms ...

epidemic occurred between 1556 and 1560, with an estimated mortality rate of 20%. *

Smallpox Smallpox was an infectious disease caused by variola virus (often called smallpox virus) which belongs to the genus Orthopoxvirus. The last naturally occurring case was diagnosed in October 1977, and the World Health Organization (WHO) c ...

killed an estimated 60 million Europeans during the 18th century (approximately 400,000 per year). Up to 30% of those infected, including 80% of the children under 5 years of age, died from the disease, and one-third of the survivors went blind. * In the 19th century,

tuberculosis Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, in ...

killed an estimated one-quarter of the adult population of Europe; by 1918 one in six deaths in France were still caused by TB. * The Influenza Pandemic of 1918 (or the

Spanish flu The 1918–1920 influenza pandemic, commonly known by the misnomer Spanish flu or as the Great Influenza epidemic, was an exceptionally deadly global influenza pandemic caused by the H1N1 influenza A virus. The earliest documented case was ...

) killed 25–50 million people (about 2% of world population of 1.7 billion). Today

Influenza Influenza, commonly known as "the flu", is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. These symptoms ...

kills about 250,000 to 500,000 worldwide each year.

Emerging diseases

In most cases, microorganisms live in harmony with their hosts via mutual or

commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit fro ...

interactions. Diseases can emerge when existing parasites become pathogenic or when new pathogenic parasites enter a new host. #

Coevolution In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well ...

between

parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

and

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places * Host, Pennsylvania, a village in Berks County People *Jim Host (born 1937), American businessman * Michel Host ...

can lead to hosts becoming resistant to the parasites or the parasites may evolve greater

virulence Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ca ...

, leading to immunopathological disease. # Human activity is involved with many

emerging infectious disease An emerging infectious disease (EID) is an infectious disease whose incidence has increased recently (in the past 20 years), and could increase in the near future. The minority that are capable of developing efficient transmission between human ...

s, such as

environmental change Environmental change is a change or disturbance of the environment most often caused by human influences and natural ecological processes. Environmental changes include various factors, such as natural disasters, human interferences, or animal in ...

enabling a parasite to occupy new niches. When that happens, a

pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

that had been confined to a remote habitat has a wider distribution and possibly a new

host organism In biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasite, parasitic, a mutualism (biology), mutualistic, or a commensalism, commensalist ''guest'' (symbiont). The guest is typically provided with ...

. Parasites jumping from nonhuman to human hosts are known as

zoonoses A zoonosis (; plural zoonoses) or zoonotic disease is an infectious disease of humans caused by a pathogen (an infectious agent, such as a bacterium, virus, parasite or prion) that has jumped from a non-human (usually a vertebrate) to a human. ...

. Under disease invasion, when a parasite invades a new host species, it may become pathogenic in the new host. Several human activities have led to the emergence of

zoonotic A zoonosis (; plural zoonoses) or zoonotic disease is an infectious disease of humans caused by a pathogen (an infectious agent, such as a bacterium, virus, parasite or prion) that has jumped from a non-human (usually a vertebrate) to a human. ...

human pathogens, including viruses, bacteria, protozoa, and rickettsia, and spread of

vector-borne In epidemiology, a disease vector is any living agent that carries and transmits an infectious pathogen to another living organism; agents regarded as vectors are organisms, such as Parasitism, parasites or Microorganism, microbes. The first major ...

diseases, see also

globalization and disease Globalization, the flow of information, goods, capital, and people across political and geographic boundaries, allows infectious diseases to rapidly spread around the world, while also allowing the alleviation of factors such as hunger and poverty ...

and

wildlife disease Wild animals, domestic animals and humans share a large and increasing number of infectious diseases, known as zoonoses. The continued globalization of society, human population growth, and associated landscape change further increase the interacti ...

: * Encroachment on wildlife

habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...

s. The construction of new villages and housing developments in rural areas force animals to live in dense populations, creating opportunities for microbes to mutate and emerge. * Changes in

agriculture Agriculture or farming is the practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that enabled people to ...

. The introduction of new crops attracts new crop pests and the microbes they carry to farming communities, exposing people to unfamiliar diseases. * The destruction of

rain forests Rainforests are characterized by a closed and continuous tree canopy, moisture-dependent vegetation, the presence of epiphytes and lianas and the absence of wildfire. Rainforest can be classified as tropical rainforest or temperate rainforest ...

. As countries make use of their rain forests, by building roads through forests and clearing areas for settlement or commercial ventures, people encounter insects and other animals harboring previously unknown microorganisms. * Uncontrolled

urbanization Urbanization (or urbanisation) refers to the population shift from rural to urban areas, the corresponding decrease in the proportion of people living in rural areas, and the ways in which societies adapt to this change. It is predominantly t ...

. The rapid growth of cities in many developing countries tends to concentrate large numbers of people into crowded areas with poor sanitation. These conditions foster transmission of contagious diseases. * Modern

transport Transport (in British English), or transportation (in American English), is the intentional movement of humans, animals, and goods from one location to another. Modes of transport include air, land (rail and road), water, cable, pipeline, an ...

. Ships and other cargo carriers often harbor unintended "passengers", that can spread diseases to faraway destinations. While with international jet-airplane travel, people infected with a disease can carry it to distant lands, or home to their families, before their first symptoms appear.

Germ theory of disease

Stamps of Germany (DDR) 1980, MiNr Zusammendruck 2534-2537

In Antiquity, the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

historian

Thucydides Thucydides (; grc, , }; BC) was an Athenian historian and general. His ''History of the Peloponnesian War'' recounts the fifth-century BC war between Sparta and Athens until the year 411 BC. Thucydides has been dubbed the father of "scientifi ...

( – ) was the first person to write, in his account of the

plague of Athens The Plague of Athens ( grc, Λοιμὸς τῶν Ἀθηνῶν}, ) was an epidemic that devastated the city-state of Athens in ancient Greece during the second year (430 BC) of the Peloponnesian War when an Athenian victory still seemed within r ...

, that diseases could spread from an infected person to others. In his ''On the Different Types of Fever'' (), the Greco-Roman physician

Galen Aelius Galenus or Claudius Galenus ( el, Κλαύδιος Γαληνός; September 129 – c. AD 216), often Anglicized as Galen () or Galen of Pergamon, was a Greek physician, surgeon and philosopher in the Roman Empire. Considered to be one of ...

speculated that plagues were spread by "certain seeds of plague", which were present in the air. In the

Sushruta Samhita The ''Sushruta Samhita'' (सुश्रुतसंहिता, IAST: ''Suśrutasaṃhitā'', literally "Suśruta's Compendium") is an ancient Sanskrit text on medicine and surgery, and one of the most important such treatises on this subj ...

, the ancient Indian physician Sushruta theorized: "Leprosy, fever, consumption, diseases of the eye, and other infectious diseases spread from one person to another by sexual union, physical contact, eating together, sleeping together, sitting together, and the use of same clothes, garlands and pastes." This book has been dated to about the sixth century BC. A basic form of contagion theory was proposed by

Persian Persian may refer to: * People and things from Iran, historically called ''Persia'' in the English language ** Persians, the majority ethnic group in Iran, not to be conflated with the Iranic peoples ** Persian language, an Iranian language of the ...

physician

Ibn Sina Ibn Sina ( fa, ابن سینا; 980 – June 1037 CE), commonly known in the West as Avicenna (), was a Persian polymath who is regarded as one of the most significant physicians, astronomers, philosophers, and writers of the Islamic G ...

(known as Avicenna in Europe) in ''

The Canon of Medicine ''The Canon of Medicine'' ( ar, القانون في الطب, italic=yes ''al-Qānūn fī al-Ṭibb''; fa, قانون در طب, italic=yes, ''Qanun-e dâr Tâb'') is an encyclopedia of medicine in five books compiled by Persian physician-phi ...

'' (1025), which later became the most authoritative medical textbook in Europe up until the 16th century. In Book IV of the ''Canon'', Ibn Sina discussed

epidemics An epidemic (from Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of patients among a given population within an area in a short period of time. Epidemics of infectious d ...

, outlining the classical

miasma theory The miasma theory (also called the miasmatic theory) is an obsolete medical theory that held that diseases—such as cholera, chlamydia, or the Black Death—were caused by a ''miasma'' (, Ancient Greek for 'pollution'), a noxious form of "bad ...

and attempting to blend it with his own early contagion theory. He mentioned that people can transmit disease to others by breath, noted contagion with

tuberculosis Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, in ...

, and discussed the transmission of disease through water and dirt. The concept of invisible contagion was later discussed by several

Islamic scholars In Islam, the ''ulama'' (; ar, علماء ', singular ', "scholar", literally "the learned ones", also spelled ''ulema''; feminine: ''alimah'' ingularand ''aalimath'' lural are the guardians, transmitters, and interpreters of religious ...

in the

Ayyubid Sultanate The Ayyubid dynasty ( ar, الأيوبيون '; ) was the founding dynasty of the medieval Sultanate of Egypt established by Saladin in 1171, following his abolition of the Fatimid Caliphate of Egypt. A Sunni Muslim of Kurdish origin, Saladin ...

who referred to them as '' najasat'' ("impure substances"). The

fiqh ''Fiqh'' (; ar, فقه ) is Islamic jurisprudence. Muhammad-> Companions-> Followers-> Fiqh. The commands and prohibitions chosen by God were revealed through the agency of the Prophet in both the Quran and the Sunnah (words, deeds, and ...

scholar

Ibn al-Haj al-Abdari Moḥammed ibn al-Hajj al-Abdari al-Fasi (or Mohammed Ibn Mohammed ibn Mohammed Abu Abdallah Ibn al-Hajj al-Abdari al-Maliki al-Fassi; ar, إبن الحاج العبدري الفاسي) also known simply as Ibn al-Haj or Ibn al-Hajj was a Morocca ...

(–1336), while discussing Islamic diet and

hygiene Hygiene is a series of practices performed to preserve health. According to the World Health Organization (WHO), "Hygiene refers to conditions and practices that help to maintain health and prevent the spread of diseases." Personal hygiene refer ...

, gave warnings about how contagion can contaminate water, food, and garments, and could spread through the water supply, and may have implied contagion to be unseen particles. When the

Black Death The Black Death (also known as the Pestilence, the Great Mortality or the Plague) was a bubonic plague pandemic occurring in Western Eurasia and North Africa from 1346 to 1353. It is the most fatal pandemic recorded in human history, causi ...

bubonic plague Bubonic plague is one of three types of plague caused by the plague bacterium (''Yersinia pestis''). One to seven days after exposure to the bacteria, flu-like symptoms develop. These symptoms include fever, headaches, and vomiting, as well a ...

reached

Al-Andalus Al-Andalus DIN 31635, translit. ; an, al-Andalus; ast, al-Ándalus; eu, al-Andalus; ber, ⴰⵏⴷⴰⵍⵓⵙ, label=Berber languages, Berber, translit=Andalus; ca, al-Àndalus; gl, al-Andalus; oc, Al Andalús; pt, al-Ândalus; es, ...

in the 14th century, the Arab physicians Ibn Khatima () and

Ibn al-Khatib Lisan ad-Din Ibn al-Khatib ( ar, لسان الدين ابن الخطيب, Lisān ad-Dīn Ibn al-Khaṭīb) (Born 16 November 1313, Loja– died 1374, Fes; full name in ar, محمد بن عبد الله بن سعيد بن عبد الله بن � ...

(1313–1374) hypothesised that infectious diseases were caused by "minute bodies" and described how they can be transmitted through garments, vessels and earrings. Ideas of contagion became more popular in Europe during the

Renaissance The Renaissance ( , ) , from , with the same meanings. is a period in European history marking the transition from the Middle Ages to modernity and covering the 15th and 16th centuries, characterized by an effort to revive and surpass ideas ...

, particularly through the writing of the Italian physician

Girolamo Fracastoro Girolamo Fracastoro ( la, Hieronymus Fracastorius; c. 1476/86 August 1553) was an Italian physician, poet, and scholar in mathematics, geography and astronomy. Fracastoro subscribed to the philosophy of atomism, and rejected appeals to hidden c ...

.

Anton van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch microbiologist and microscopist in the Golden Age of Dutch science and technology. A largely self-taught man in science, he is commonly known as " the F ...

(1632–1723) advanced the science of

microscopy Microscopy is the technical field of using microscopes to view objects and areas of objects that cannot be seen with the naked eye (objects that are not within the resolution range of the normal eye). There are three well-known branches of micr ...

by being the first to observe microorganisms, allowing for easy visualization of bacteria. In the mid-19th century

John Snow John Snow (15 March 1813 – 16 June 1858) was an English physician and a leader in the development of anaesthesia and medical hygiene. He is considered one of the founders of modern epidemiology, in part because of his work in tracing the so ...

and

William Budd William Budd (14 September 1811 – 9 January 1880) was an English physician and epidemiologist known for recognizing that infectious diseases were contagious. He recognized that the "poisons" involved in infectious diseases multiplied in the int ...

did important work demonstrating the contagiousness of typhoid and cholera through contaminated water. Both are credited with decreasing epidemics of cholera in their towns by implementing measures to prevent contamination of water.

Louis Pasteur Louis Pasteur (, ; 27 December 1822 – 28 September 1895) was a French chemist and microbiologist renowned for his discoveries of the principles of vaccination, microbial fermentation and pasteurization, the latter of which was named afte ...

proved beyond doubt that certain diseases are caused by infectious agents, and developed a vaccine for

rabies Rabies is a viral disease that causes encephalitis in humans and other mammals. Early symptoms can include fever and tingling at the site of exposure. These symptoms are followed by one or more of the following symptoms: nausea, vomiting, vi ...

.

Robert Koch Heinrich Hermann Robert Koch ( , ; 11 December 1843 – 27 May 1910) was a German physician and microbiologist. As the discoverer of the specific causative agents of deadly infectious diseases including tuberculosis, cholera (though the Vibrio ...

provided the study of infectious diseases with a scientific basis known as

Koch's postulates Koch's postulates ( )"Koch"
''

Edward Jenner Edward Jenner, (17 May 1749 – 26 January 1823) was a British physician and scientist who pioneered the concept of vaccines, and created the smallpox vaccine, the world's first vaccine. The terms ''vaccine'' and ''vaccination'' are derived f ...

,

Jonas Salk Jonas Edward Salk (; born Jonas Salk; October 28, 1914June 23, 1995) was an American virologist and medical researcher who developed one of the first successful polio vaccines. He was born in New York City and attended the City College of New ...

and

Albert Sabin Albert Bruce Sabin ( ; August 26, 1906 – March 3, 1993) was a Polish-American medical researcher, best known for developing the oral polio vaccine, which has played a key role in nearly eradicating the disease. In 1969–72, he served as the ...

developed effective vaccines for

smallpox Smallpox was an infectious disease caused by variola virus (often called smallpox virus) which belongs to the genus Orthopoxvirus. The last naturally occurring case was diagnosed in October 1977, and the World Health Organization (WHO) c ...

and

polio Poliomyelitis, commonly shortened to polio, is an infectious disease caused by the poliovirus. Approximately 70% of cases are asymptomatic; mild symptoms which can occur include sore throat and fever; in a proportion of cases more severe s ...

, which would later result in the eradication and near-eradication of these diseases, respectively.

Alexander Fleming Sir Alexander Fleming (6 August 1881 – 11 March 1955) was a Scottish physician and microbiologist, best known for discovering the world's first broadly effective antibiotic substance, which he named penicillin. His discovery in 1928 of w ...

discovered the world's first

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of ...

,

penicillin Penicillins (P, PCN or PEN) are a group of β-lactam antibiotics originally obtained from ''Penicillium'' moulds, principally '' P. chrysogenum'' and '' P. rubens''. Most penicillins in clinical use are synthesised by P. chrysogenum using ...

, which Florey and Chain then developed.

Gerhard Domagk Gerhard Johannes Paul Domagk (; 30 October 1895 – 24 April 1964) was a German pathologist and bacteriologist. He is credited with the discovery of sulfonamidochrysoidine (KL730) as an antibiotic for which he received the 1939 Nobel Prize in Phy ...

developed

sulphonamides In organic chemistry, the sulfonamide functional group (also spelled sulphonamide) is an organosulfur group with the structure . It consists of a sulfonyl group () connected to an amine group (). Relatively speaking this group is unreactive. B ...

, the first broad spectrum synthetic antibacterial drugs.

Medical specialists

The

medical treatment A therapy or medical treatment (often abbreviated tx, Tx, or Tx) is the attempted remediation of a health problem, usually following a medical diagnosis. As a rule, each therapy has indications and contraindications. There are many different ...

of infectious diseases falls into the

medical field A medical specialty is a branch of medical practice that is focused on a defined group of patients, diseases, skills, or philosophy. Examples include those branches of medicine that deal exclusively with children (paediatrics), cancer (oncology), ...

of

Infectious Disease An infection is the invasion of tissues by pathogens, their multiplication, and the reaction of host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmissible disease or communicable dise ...

and in some cases the study of propagation pertains to the field of

Epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and shapes policy decisions and evidenc ...

. Generally, infections are initially diagnosed by

primary care Primary care is the day-to-day healthcare given by a health care provider. Typically this provider acts as the first contact and principal point of continuing care for patients within a healthcare system, and coordinates other specialist care t ...

physicians or internal medicine specialists. For example, an "uncomplicated"

pneumonia Pneumonia is an inflammatory condition of the lung primarily affecting the small air sacs known as alveoli. Symptoms typically include some combination of productive or dry cough, chest pain, fever, and difficulty breathing. The severity ...

will generally be treated by the internist or the

pulmonologist Pulmonology (, , from Latin ''pulmō, -ōnis'' "lung" and the Greek suffix "study of"), pneumology (, built on Greek πνεύμων "lung") or pneumonology () is a medical specialty that deals with diseases involving the respiratory tract.

(lung physician). The work of the infectious diseases specialist therefore entails working with both patients and general practitioners, as well as laboratory scientists, immunologists,

bacteriologist A bacteriologist is a microbiologist, or similarly trained professional, in bacteriology -- a subdivision of microbiology that studies bacteria, typically Pathogenic bacteria, pathogenic ones. Bacteriologists are interested in studying and learnin ...

s and other specialists. An infectious disease team may be alerted when: * The disease has not been definitively diagnosed after an initial workup * The patient is

immunocompromised Immunodeficiency, also known as immunocompromisation, is a state in which the immune system's ability to fight infectious diseases and cancer is compromised or entirely absent. Most cases are acquired ("secondary") due to extrinsic factors that a ...

(for example, in

AIDS Human immunodeficiency virus infection and acquired immunodeficiency syndrome (HIV/AIDS) is a spectrum of conditions caused by infection with the human immunodeficiency virus (HIV), a retrovirus. Following initial infection an individual m ...

or after

chemotherapy Chemotherapy (often abbreviated to chemo and sometimes CTX or CTx) is a type of cancer treatment that uses one or more anti-cancer drugs (chemotherapeutic agents or alkylating agents) as part of a standardized chemotherapy regimen. Chemotherap ...

); * The

infectious agent In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...

is of an uncommon nature (e.g.

tropical disease Tropical diseases are Infectious disease, diseases that are prevalent in or unique to tropics, tropical and subtropics, subtropical regions. The diseases are less prevalent in temperate climates, due in part to the occurrence of a cold season, whic ...

s); * The disease has not responded to first line

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of ...

s; * The disease might be dangerous to other patients, and the patient might have to be isolated

Society and culture

Several studies have reported associations between pathogen load in an area and human behavior. Higher pathogen load is associated with decreased size of ethnic and religious groups in an area. This may be due high pathogen load favoring avoidance of other groups, which may reduce pathogen transmission, or a high pathogen load preventing the creation of large settlements and armies that enforce a common culture. Higher pathogen load is also associated with more restricted sexual behavior, which may reduce pathogen transmission. It also associated with higher preferences for health and attractiveness in mates. Higher

fertility rate The total fertility rate (TFR) of a population is the average number of children that would be born to a woman over her lifetime if: # she were to experience the exact current age-specific fertility rates (ASFRs) through her lifetime # she were t ...

s and shorter or less parental care per child is another association that may be a compensation for the higher mortality rate. There is also an association with

polygyny Polygyny (; from Neoclassical Greek πολυγυνία (); ) is the most common and accepted form of polygamy around the world, entailing the marriage of a man with several women. Incidence Polygyny is more widespread in Africa than in any o ...

which may be due to higher pathogen load, making selecting males with a high genetic resistance increasingly important. Higher pathogen load is also associated with more collectivism and less individualism, which may limit contacts with outside groups and infections. There are alternative explanations for at least some of the associations although some of these explanations may in turn ultimately be due to pathogen load. Thus, polygyny may also be due to a lower male: female ratio in these areas but this may ultimately be due to male infants having increased mortality from infectious diseases. Another example is that poor socioeconomic factors may ultimately in part be due to high pathogen load preventing economic development.

Fossil record

Evidence of infection in fossil remains is a subject of interest for

paleopathologists Paleopathology, also spelled palaeopathology, is the study of ancient diseases and injuries in organisms through the examination of fossils, mummified tissue, skeletal remains, and analysis of coprolites. Specific sources in the study of ancient ...

, scientists who study occurrences of injuries and illness in extinct life forms. Signs of infection have been discovered in the bones of carnivorous dinosaurs. When present, however, these infections seem to tend to be confined to only small regions of the body. A skull attributed to the early carnivorous dinosaur ''

Herrerasaurus ischigualastensis ''Herrerasaurus'' is a genus of saurischian dinosaur from the Late Triassic period. This genus was one of the earliest dinosaurs from the fossil record. Its name means "Herrera's lizard", after the rancher who discovered the first specimen in ...

'' exhibits pit-like wounds surrounded by swollen and porous bone. The unusual texture of the bone around the wounds suggests they were affected by a short-lived, non-lethal infection. Scientists who studied the skull speculated that the bite marks were received in a fight with another ''Herrerasaurus''. Other carnivorous dinosaurs with documented evidence of infection include ''

Acrocanthosaurus ''Acrocanthosaurus'' ( ; ) is a genus of carcharodontosaurid dinosaur that existed in what is now North America during the Aptian and early Albian stages of the Early Cretaceous, from 113 to 110 million years ago. Like most dinosaur genera, ' ...

'', ''

Allosaurus ''Allosaurus'' () is a genus of large carnosaurian theropod dinosaur that lived 155 to 145 million years ago during the Late Jurassic epoch ( Kimmeridgian to late Tithonian). The name "''Allosaurus''" means "different lizard" alludin ...

'', ''

Tyrannosaurus ''Tyrannosaurus'' is a genus of large theropoda, theropod dinosaur. The species ''Tyrannosaurus rex'' (''rex'' meaning "king" in Latin), often called ''T. rex'' or colloquially ''T-Rex'', is one of the best represented theropods. ''Tyrannosa ...

'' and a tyrannosaur from the

Kirtland Formation The Kirtland Formation (originally the Kirtland Shale) is a sedimentary geological formation. Description The Kirtland Formation is the product of alluvial muds and overbank sand deposits from the many channels draining the coastal plain that ...

. The infections from both tyrannosaurs were received by being bitten during a fight, like the ''Herrerasaurus'' specimen.Molnar, R. E., 2001, "Theropod paleopathology: a literature survey": In: ''Mesozoic Vertebrate Life'', edited by Tanke, D. H., and Carpenter, K., Indiana University Press, pp. 337–63.

Outer space

A 2006

Space Shuttle The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program na ...

experiment found that ''

Salmonella typhimurium ''Salmonella enterica'' subsp. ''enterica'' is a subspecies of ''Salmonella enterica'', the rod-shaped, flagellated, aerobic, Gram-negative bacterium. Many of the pathogenic serovars of the ''S. enterica'' species are in this subspecies, includi ...

'', a bacterium that can cause

food poisoning Foodborne illness (also foodborne disease and food poisoning) is any illness resulting from the spoilage of contaminated food by pathogenic bacteria, viruses, or parasites that contaminate food, as well as prions (the agents of mad cow disease) ...

, became more

virulent Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its ability to ca ...

when cultivated in

space Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consider ...

. On April 29, 2013, scientists in

Rensselaer Polytechnic Institute Rensselaer Polytechnic Institute () (RPI) is a private research university in Troy, New York, with an additional campus in Hartford, Connecticut. A third campus in Groton, Connecticut closed in 2018. RPI was established in 1824 by Stephen Van ...

, funded by

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

, reported that, during

spaceflight Spaceflight (or space flight) is an application of astronautics to fly spacecraft into or through outer space, either with or without humans on board. Most spaceflight is uncrewed and conducted mainly with spacecraft such as satellites in or ...

on the

International Space Station The International Space Station (ISS) is the largest modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos (Russia), JAXA ...

,

microbes A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

seem to adapt to the space environment in ways "not observed on Earth" and in ways that "can lead to increases in growth and virulence". More recently, in 2017,

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

were found to be more resistant to

antibiotic An antibiotic is a type of antimicrobial substance active against bacteria. It is the most important type of antibacterial agent for fighting bacterial infections, and antibiotic medications are widely used in the treatment and prevention of ...

s and to thrive in the near-weightlessness of space.

Microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

s have been observed to survive the vacuum of outer space.

References

External links

European Center for Disease Prevention and Control

U.S. Centers for Disease Control and Prevention

Infectious Disease Society of America
(IDSA)
Infectious Disease Index
of the Public Health Agency of Canada (PHAC)
Vaccine Research Center
Information concerning vaccine research clinical trials for Emerging and re-Emerging Infectious Diseases. *
Microbes & Infection
' (journal)
Table: Global deaths from communicable diseases, 2010
– Canadian Broadcasting Corp. {{DEFAULTSORT:Infection Infectious diseases, Epidemiology

= Primary pathogens

= Opportunistic pathogens

= Secondary infection

= Other types of infection

Infectious or not

Contagiousness

By anatomic location

Prevention

Immunity

Host genetic factors

Treatments

Epidemiology

Historic pandemics

Emerging diseases

Germ theory of disease

Medical specialists

Society and culture

Fossil record

Outer space

See also

References

External links