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An infection is the invasion of tissues by
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, their multiplication, and the reaction of
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 ( ...
tissues to the infectious agent and the
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 (1 ...
s they produce. An infectious disease, also known as a transmissible disease or communicable disease, is an
illness A disease is a particular abnormal condition that negatively affects the structure or function (biology), function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medica ...
resulting from an infection. Infections can be caused by a wide range of
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, most prominently
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 micrometre The micrometre (Amer ...
and
virus A virus is a wikt:submicroscopic, submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and ...
es. Hosts can fight infections using their
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 Tumor immunology, cancer cells and objects such ...
.
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), fu ...
ian hosts react to infections with an
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 old ...
, 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''.


= 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 (biology), host. The Pathogen#Pathogenicity, pathogen ...
(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).
Opportunistic infection An opportunistic infection is an infection caused by pathogens (bacteria, fungus, fungi, Parasitism, parasites or viruses) that take advantage of an opportunity not normally available. These opportunities can stem from a variety of sources, such a ...
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 The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongu ...
or the
upper respiratory tract The respiratory tract is the subdivision of the respiratory system involved with the process of Respiration (physiology), respiration in mammals. The respiratory tract is lined with respiratory epithelium as respiratory mucosa. Air is breathed ...
, 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 (anatomy), colon). Colitis may be acute (medicine), acute and self-limited or chronic condition, long-term. It broadly fits into the category of digestive diseases. In a medical co ...
) 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 pa ...
wound infections or compound fractures). An opportunistic disease requires impairment of host defenses, which may occur as a result of
genetic defect A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorders ...
s (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 genetic disorder, hereditary diseases in which certain cells of the immune system have difficulty ...
), 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 Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Poss ...
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 (list of chemotherapeutic agents, chemotherapeutic agents or alkylating agents) as part of a standardized ...
), exposure to
ionizing radiation Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionization, ionize atoms or molecules by detaching electrons from them. Some particles ...
, 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 morbillivirus, 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 ...
,
malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects humans and other animals. Malaria causes Signs and symptoms, symptoms that typically include fever, fatigue (medical), tiredness, vomiting, and headaches. In se ...
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 of an individual's current health problem, a secondary infection is a sequela 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 (biology), tissue of the body, creating a deep but relatively narrow entry wound. In contrast, a blunt trauma, blunt or ''non-penetrating'' ...
(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 Accident, unintentionally and may be caused by blunt trauma, penetrating trauma, burning, Toxin, toxic exposure, asp ...
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 (anatomy), appendix. Symptoms commonly include right lower abdominal pain, nausea, vomiting, and anorexia (symptom), decreased appetite. However, approximately 40% of people do not have these typical ...
, 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"
''Random House Webster's Unabridged Dictionary''.
are four criteria designed to es ...
(first proposed by
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 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 Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biol ...
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 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, and ...
, 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 Homology (biology), homologous to the female ovary. The functions of the testes are to produce both sperm and androgens, pr ...
. 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 risk factor, determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and shapes policy decision ...
, 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 Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and risk factor, determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and sha ...
is sporadic, or just an occasional occurrence;
endemic Endemism is the state of a species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of or ...
, 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 Freedom of movement, movement of people, animals and goods which is intended to prevent the spread of disease or Pest (organism), pests. It is often used in connection to disease and illness, preventing th ...
) 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 organ (biology), 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 Tissue (biology) ...
infected, including: *
Urinary tract infection A urinary tract infection (UTI) is an infection An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they p ...
*
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 ( ...
* Odontogenic infection (an infection that originates within a
tooth A tooth (plural, : teeth) is a hard, calcification, calcified structure found in the jaws (or mouths) of many vertebrates and used to Mastication, break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with ...
or in the closely surrounding tissues) * Vaginal infections * Intra-amniotic infection In addition, locations of
inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and mol ...
where infection is the most common cause include
pneumonia Pneumonia is an Inflammation, inflammatory condition of the lung primarily affecting the small air sacs known as Pulmonary alveolus, alveoli. Symptoms typically include some combination of phlegm, productive or dry cough, chest pain, fever, ...
,
meningitis Meningitis is Acute (medical), acute or Chronic (medical), 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. Ot ...
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, and ...
.


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 Pathogen, disease-causing micro-organisms (such as pathogenic bacteria, viruses, pathogenic fungi, and parasitism, parasites). There are two categories of asepsis: medical and surgical. The modern day notio ...
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 contraception, barrier device used during sexual intercourse to reduce the probability of pregnancy or a Sexually transmitted disease, sexually transmitted infection (STI). There are both male and female con ...
, 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 biological tissue, tissue/skin to reduce the possibility of infection ...
, 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 endospore, bacterial spores; it is less effecti ...
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 pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy, ...
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 (biology), host. The Pathogen#Pathogenicity, pathogen ...
, 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 acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immun ...
(
HIV 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 ...
) 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 Graph (discrete mathematics), 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 ...
. 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 virus in 2001. A general method to prevent transmission of vector-borne pathogens is
pest control Pest control is the regulation or management of a species defined as a pest (organism), pest; any animal, plant or fungus that impacts adversely on human activities or environment. The human response depends on the importance of the damage don ...
. In cases where infection is merely suspected, individuals may be quarantined 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 may be imposed to prevent infection from spreading beyond the community, or in the case of protective sequestration, 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 intervention (epidemiology), non-pharmaceutical interventions or measures intend ...
, 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 of the pathogen. Specific acquired immunity against infectious diseases may be mediated by
antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform ...
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 ** Complement (music)#Aggregate complementation, Aggregate c ...
-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 i ...
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 temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Th ...
and
inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and mol ...
, and has the potential to be more devastating than direct damage caused by a microbe. Resistance to infection ( immunity) 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 pathogens. 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 bond, glycosidic linkages. This carbohydrate c ...
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 or Pegylated interferon-alpha-2b (brand names Pegasys or PEG-Intron) combined with
ribavirin Ribavirin, also known as tribavirin, is an antiviral medication used to treat Human respiratory syncytial virus, RSV infection, hepatitis C and some viral hemorrhagic fevers. For hepatitis C, it is used in combination with other medications suc ...
, 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 pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy, ...
; 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 Cryptoc ...
and
antiparasitic Antiparasitics are a class of medications which are indicated for the treatment of parasitic diseases, such as those caused by helminths, amoeba, parasitic nutrition#Ectoparasitism, ectoparasites, Microsporum, parasitic fungi, and protozoa, among o ...
(including antiprotozoal and
antihelminthic Anthelmintics or antihelminthics are a group of antiparasitic, 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 h ...
) 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 surface area, body surfaces such as the human skin, skin or mucous membranes to treat ailments ...
ly. Severe infections of the
brain A brain is an organ (biology), 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 Visual perception, vision. I ...
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 beta-lactam antibiotic, β-lactam antibiotics originally obtained from ''Penicillium'' Mold (fungus), moulds, principally ''Penicillium chrysogenum, P. chrysogenum'' and ''Penicillium rubens, P. ru ...
,
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 chemistry, medicinal and bacteriology, bacteriologic category of traditional Gram-negative antibacterial medications that inhibit protein synthesis and contain as a portion of the molecule an amino-modified glycoside ...
s,
macrolide The Macrolides are a class of natural products that consist of a large macrocycle, 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-memb ...
s, quinolones 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 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

In 2010, about 10 million people died of infectious diseases. The
World Health Organization The World Health Organization (WHO) is a list of specialized agencies of the United Nations, specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainme ...
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 AIDS, acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the ...
/
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 ma ...
, TB and
malaria Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects humans and other animals. Malaria causes Signs and symptoms, symptoms that typically include fever, fatigue (medical), tiredness, vomiting, and headaches. In se ...
. 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 o ...
,
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 bacteria, bacterium ''Corynebacterium diphtheriae''. Most infections are asymptomatic or have a mild Course (medicine), clinical course, but in some outbreaks more than 10% of those diagnosed with the di ...
,
measles Measles is a highly contagious infectious disease caused by Measles morbillivirus, 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 ...
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 usual ...
. 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 invention of writing systems is considered prehistory. "History" is an umbrella term comprising past events as we ...
. 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''. T ...
, 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 North Africa, or Northern Africa is a region encompassing the northern portio ...
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 Ali Maow Maalin#Maalin's case, last naturally occurring case was diagnosed in October 1977, and the World ...
, 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 language, Spanish: México), officially the United Mexican States, is a List of sovereign states, country in the southern portion of North America. It is borders of Mexico, bordered to the north by the United States; to the so ...
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 Ali Maow Maalin#Maalin's case, last naturally occurring case was diagnosed in October 1977, and the World ...
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 influenza A virus subtype H1N1, H1N1 influenza A virus. Th ...
) 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 (biology), mutualism, in which both o ...
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 Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...
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 (biology), host. The Pathogen#Pathogenicity, pathogen ...
, leading to immunopathological disease. # Human activity is involved with many emerging infectious diseases, such as
environmental change Environmental change is a change or disturbance (ecology), disturbance of the environment (biophysical), environment most often caused by human impact on the environment, human influences and natural ecosystem ecology, ecological processes. Environm ...
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 Cross-species transmission, jumped from a non-human (usuall ...
. 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 Cross-species transmission, jumped from a non-human (usuall ...
human pathogens, including viruses, bacteria, protozoa, and rickettsia, and spread of vector-borne diseases, see also globalization and disease and wildlife disease: * 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 Plant, plants and livestock. Agriculture was the key development in the rise of Sedentism, sedentary human civilization, whereby farming of Domestication, domesticated species created food ...
. 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 Tropical rain ...
. 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 area, 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 predo ...
. 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 Motion, movement of humans, animals, and cargo, goods from one location to another. Mode of transport, Modes of transport include aviation, air, land ...
. 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

In
Antiquity Antiquity or Antiquities may refer to: Historical objects or periods Artifacts *Antiquities Antiquities are objects from Ancient history, antiquity, especially the civilizations of the Mediterranean: the Classical antiquity of Greece and Ro ...
, 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 Classical Athens, Athenian historian and general. His ''History of the Peloponnesian War'' recounts Peloponnesian War, the fifth-century BC war between Sparta and Athens until the year 411 BC. Thucydides has ...
( – ) 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 History of Athens, Athens in ancient Greece during the second year (430 BC) of the Peloponnesian War when an Athenian victory st ...
, 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 Anglicization, Anglicized as Galen () or Galen of Pergamon, was a Ancient Greeks, Greek physician, surgeon and Philosophy, philosopher i ...
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 subje ...
, the ancient Indian physician
Sushruta Sushruta, or ''Suśruta'' (Sanskrit: सुश्रुत, IAST: , ) was an ancient Indian physician. The ''Sushruta Samhita'' (''Sushruta's Compendium''), a treatise ascribed to him, is one of the most important surviving ancient treatises on ...
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 physician
Ibn Sina Ibn Sina ( fa, ابن سینا; 980 – June 1037 CE), commonly known in the West as Avicenna (), was a Persians, Persian polymath who is regarded as one of the most significant physicians, astronomers, philosophers, and writers of the ...
(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 Medical literature, encyclopedia of medicine in five books compiled by Pe ...
'' (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 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 ...
, 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 Ancient Greek includes the ...
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 Islam (; ar, ۘالِإسلَام, , ) is an Abrahamic monotheistic religion centred primarily around the Quran, a religious text considered by Muslims to be the direct word of God in Islam, God (or ''Allah'') as it wa ...
in the Ayyubid Sultanate who referred to them as '' najasat'' ("impure substances"). The
fiqh ''Fiqh'' (; ar, فقه ) is Islamic jurisprudence. Muhammad-> Sahabah, Companions-> Tabi‘un, Followers-> Fiqh. The commands and prohibitions chosen by God were revealed through the agency of the Prophet in both the Quran and the Sunnah (wo ...
scholar Ibn al-Haj al-Abdari (–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 North Africa, or Northern Africa is a region encompassing the northern portio ...
bubonic plague Bubonic plague is one of three types of Plague (disease), plague caused by the plague Bacteria, bacterium (''Yersinia pestis''). One to seven days after exposure to the bacteria, flu-like symptoms develop. These symptoms include fever, headac ...
reached
Al-Andalus Al-Andalus translit. ; an, al-Andalus; ast, al-Ándalus; eu, al-Andalus; ber, ⴰⵏⴷⴰⵍⵓⵙ, label=Berber, translit=Andalus; ca, al-Àndalus; gl, al-Andalus; oc, Al Andalús; pt, al-Ândalus; es, al-Ándalus () was the Mus ...
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 س ...
(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 Periodization, period in History of Europe, European history marking the transition from the Middle Ages to modernity and covering the 15th and 16th centuries, characterized by an e ...
, particularly through the writing of the Italian physician
Girolamo Fracastoro Girolamo Fracastoro ( la, Hieronymus Fracastorius; c. 1476/86 August 1553) was an Italians, Italian physician, poet, and scholar in mathematics, geography and astronomy. Fracastoro subscribed to the philosophy of atomism, and rejected appeals to ...
.
Anton van Leeuwenhoek Antonie Philips van Leeuwenhoek ( ; ; 24 October 1632 – 26 August 1723) was a Dutch Republic, Dutch microbiology, microbiologist and microscopist in the Dutch Golden Age, Golden Age of Dutch science and technology. A largely self-taught ...
(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 Hygiene#Medical hygiene, medical hygiene. He is considered one of the founders of modern epidemiology, in part because of h ...
and William Budd 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, Fermentation, microbial fermentation and pasteurization, the latter of which wa ...
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 Encephalitis is inflammation of the brain. The severity can be variable with symptoms including reduction or alteration in consciousness, headache, fever, confusion, a stiff neck, a ...
.
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"
''Random House Webster's Unabridged Dictionary''.
are four criteria designed to es ...
.
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 Virology, virologist and medical researcher who developed one of the first successful polio vaccines. He was born in New York City and attended the City Colleg ...
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 Poliomyelitis eradication, eradicating the disease. In 1 ...
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 Ali Maow Maalin#Maalin's case, last naturally occurring case was diagnosed in October 1977, and the World ...
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 sym ...
, 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 what ...
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 pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy, ...
,
penicillin Penicillins (P, PCN or PEN) are a group of beta-lactam antibiotic, β-lactam antibiotics originally obtained from ''Penicillium'' Mold (fungus), moulds, principally ''Penicillium chrysogenum, P. chrysogenum'' and ''Penicillium rubens, P. ru ...
, 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 Sulfonamide (medicine), sulfonamidochrysoidine (KL730) as an antibiotic for which he received the ...
developed sulphonamides, 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 indication (medicine), indications and contraindications. Th ...
of infectious diseases falls into the medical field of
Infectious Disease An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An infectious disease, also known as a transmiss ...
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 risk factor, determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and shapes policy decision ...
. 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 car ...
physicians or internal medicine specialists. For example, an "uncomplicated"
pneumonia Pneumonia is an Inflammation, inflammatory condition of the lung primarily affecting the small air sacs known as Pulmonary alveolus, alveoli. Symptoms typically include some combination of phlegm, productive or dry cough, chest pain, fever, ...
will generally be treated by the internist or the pulmonologist (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 ones. Bacteriologists are interested in studying and learning about bacteria ...
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 ma ...
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 (list of chemotherapeutic agents, chemotherapeutic agents or alkylating agents) as part of a standardized ...
); * The infectious agent 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 pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy, ...
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, 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 theropoda, theropod dinosaur that lived 155 to 145 million years ago during the Late Jurassic Geologic time scale, epoch (Kimmeridgian to late Tithonian). The name "''Allosaurus''" means ...
'', ''
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 rock, sedimentary geological formation. Description The Kirtland Formation is the product of alluvial muds and overbank sand deposits from the many channels draining the ...
. 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 launch system, 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. I ...
experiment found that ''
Salmonella typhimurium ''Salmonella enterica'' subsp. ''enterica'' is a subspecies In Taxonomy (biology), biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical ...
'', a bacterium that can cause
food poisoning Foodborne illness (also foodborne disease and food poisoning) is any Disease, illness resulting from the spoilage of food contaminant, contaminated food by pathogenic bacteria, viruses, or parasites that contaminate food, as well as prions (the ...
, became more
virulent Virulence is 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 infecti ...
when cultivated in
space Space is the boundless Three-dimensional space, three-dimensional extent in which Physical body, objects and events have relative position (geometry), position and direction (geometry), direction. In classical physics, physical space is often ...
. On April 29, 2013, scientists in
Rensselaer Polytechnic Institute Rensselaer Polytechnic Institute () (RPI) is a private university, private research university in Troy, New York, with an additional campus in Rensselaer at Hartford, Hartford, Connecticut. A third campus in Groton, Connecticut closed in 2018. ...
, funded by
NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
, reported that, during
spaceflight Spaceflight (or space flight) is an application of astronautics to fly spacecraft A spacecraft is a vehicle or machine designed to spaceflight, fly in outer space. A type of artificial satellite, spacecraft are used for a variety of p ...
on the
International Space Station The International Space Station (ISS) is the largest Modular design, modular space station currently in low Earth orbit. It is a multinational collaborative project involving five participating space agencies: NASA (United States), Roscosmos ( ...
,
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 micrometre The micrometre (Amer ...
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 pathogenic bacteria, bacterial infections, and antibiotic medications are widely used in the therapy, ...
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 A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "Void (astronomy), void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Ph ...
of outer space.


See also

* Bioinformatics Resource Centers for Infectious Diseases *
Biological hazard A biological hazard, or biohazard, is a biological substance that poses a threat to the health of living organisms, primarily humans. This could include a sample of a microorganism, virus A virus is a wikt:submicroscopic, submicros ...
*
Blood-borne disease A blood-borne disease is a disease that can be spread through contamination by blood and other body fluids. Blood can contain pathogens of various types, chief among which are microorganisms, like bacteria and human parasite, parasites, and non-l ...
*
Coinfection Coinfection is the simultaneous infection An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they produce. An i ...
* Copenhagen Consensus * Cordon sanitaire * Disease diffusion mapping * Epidemiological transition *
Foodborne illness Foodborne illness (also foodborne disease and food poisoning) is any Disease, illness resulting from the spoilage of food contaminant, contaminated food by pathogenic bacteria, viruses, or parasites that contaminate food, as well as prions (the ...
*
Gene therapy Gene therapy is a medical field which focuses on the genetic modification of cells to produce a therapeutic effect or the treatment of disease by repairing or reconstructing defective genetic material. The first attempt at modifying human D ...
*
History of medicine The history of medicine is both a study of medicine throughout history as well as a multidisciplinary field of study that seeks to explore and understand medical practices, both past and present, throughout human societies. More than just histo ...
*
Hospital-acquired infection A hospital-acquired infection, also known as a nosocomial infection (from the Greek , meaning "hospital"), is an infection that is acquired in a hospital or other health care facility. To emphasize both hospital and nonhospital settings, it is so ...
*
Eradication of infectious diseases Eradication is the reduction of an infectious disease An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins the ...
*
Human Microbiome Project The Human Microbiome Project (HMP) was a United States National Institutes of Health (NIH) research initiative to improve understanding of the Human microbiota, microbiota involved in human health and disease. Launched in 2007, the first phase (H ...
*
Infection control Infection prevention and control is the discipline concerned with preventing healthcare-associated infections; a practical rather than academic sub-discipline of epidemiology. In Northern Europe, infection prevention and control is expanded from ...
*
Isolation (health care) In health care facilities, isolation represents one of several measures that can be taken to implement in infection control: the prevention of communicable diseases from being transmitted from a patient to other patients, health care workers, ...
* List of bacterial vaginosis microbiota *
List of causes of death by rate The following is a list of the causes of human deaths worldwide for different years arranged by their associated mortality rate Mortality rate, or death rate, is a measure of the number of deaths (in general, or due to a specific cause) in a ...
* List of diseases caused by insects * List of epidemics * List of infectious diseases * Mathematical modelling of infectious disease *
Multiplicity of infection In microbiology, the multiplicity of infection or MOI is the ratio of agents (e.g. phage or more generally virus, bacteria) to infection targets (e.g. Cell (biology), cell). For example, when referring to a group of cells inoculated with virus part ...
*
Neglected tropical diseases Neglected tropical diseases (NTDs) are a diverse group of tropical disease, tropical infections that are common in low-income populations in Developing country, developing regions of Africa, Asia, and the Americas. They are caused by a variety ...
* Sentinel surveillance *
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 intervention (epidemiology), non-pharmaceutical interventions or measures intend ...
* Spatiotemporal Epidemiological Modeler (STEM) * Spillover infection * Threshold host density *
Transmission (medicine) In medicine, public health, and biology, transmission is the passing of a pathogen causing Infectious disease, communicable disease from an infected host (biology), host individual or group to a particular individual or group, regardless of wheth ...
* Ubi pus, ibi evacua (Latin: "where there is pus, there evacuate it") *
Vaccine-preventable diseases A vaccine-preventable disease is an infectious disease An infection is the invasion of tissue (biology), tissues by pathogens, their multiplication, and the reaction of host (biology), host tissues to the infectious agent and the toxins they pr ...
*
Waterborne diseases Waterborne diseases are conditions (meaning adverse effects on human health, such as death, disability, illness or disorders) caused by pathogenic microorganism, micro-organisms that are transmitted in water. These diseases can be spread while ba ...


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 Epidemiology