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An infection is the invasion of tissues by
pathogen
In biology, a pathogen (, "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 theory of d ...
s, their multiplication, and the reaction of host tissues to the infectious agent and the toxin
A toxin is a naturally occurring poison produced by metabolic activities of living cells or organisms. They occur especially as proteins, often conjugated. The term was first used by organic chemist Ludwig Brieger (1849–1919), derived ...
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 adversely affects the structure or function (biology), function of all or part of an organism and is not immediately due to any external injury. Diseases are often known to be medical condi ...
resulting from an infection.
Infections can be caused by a wide range of pathogen
In biology, a pathogen (, "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 theory of d ...
s, most prominently bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
and virus
A virus is a 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 archaea. Viruses are ...
es. Hosts can fight infections using their immune system
The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
s. Mammal
A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
ian hosts react to infections with an innate
{{Short pages monitor
Among the many varieties of
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
Techniques like hand washing, wearing gowns, and wearing face masks can help prevent infections from being passed from one person to another. Aseptic technique was introduced in medicine and surgery in the late 19th century and greatly reduced the incidence of infections caused by surgery. Frequent
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
An estimated 1,680 million people died of infectious diseases in the 20th century and about 10 million in 2010.
The
With their potential for unpredictable and explosive impacts, infectious diseases have been major actors in human
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'' 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'', ''
European Center for Disease Prevention and Control
U.S. Centers for Disease Control and Prevention
Infectious Disease Society of America
(IDSA)
Vaccine Research Center
Information concerning vaccine research clinical trials for Emerging and re-Emerging Infectious Diseases. *
Microbes & Infection
' (journal) {{DEFAULTSORT:Infection Epidemiology
microorganisms
A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...
, relatively few cause disease in otherwise healthy individuals.This section incorporatepublic 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 (, "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 theory of d ...
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 intrinsicvirulence
Virulence is a pathogen's or microorganism's ability to cause damage to a host.
In most cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its abili ...
(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 immunocompromise, 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 aff ...
) or if they have unusual access to the inside of the body (for example, via trauma). Opportunistic infection
An opportunistic infection is an infection that occurs most commonly in individuals with an immunodeficiency disorder and acts more severe on those with a weakened immune system. These types of infections are considered serious and can be caused b ...
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 many are Probiotic, beneficial but others can cause infectious diseases. The nu ...
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 tract is the largest of the body's systems, after the cardiovascular system. ...
or the upper respiratory tract, and they may also result from (otherwise innocuous) microbes acquired from other hosts (as in '' Clostridioides difficile'' colitis) or from the environment as a result of traumatic introduction (as in surgical
Surgery is a medical specialty that uses manual and instrumental techniques to diagnose or treat pathological conditions (e.g., trauma, disease, injury, malignancy), to alter bodily functions (e.g., malabsorption created by bariatric surgery ...
wound infections or compound fracture An open fracture, also called a compound fracture, is a type of bone fracture (broken bone) that has an open wound in the skin near the fractured bone. The skin wound is usually caused by the bone breaking through the surface of the skin. An open fr ...
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 react ...
), exposure to antimicrobial
An antimicrobial is an agent that kills microorganisms (microbicide) or stops their growth (bacteriostatic agent). Antimicrobial medicines can be grouped according to the microorganisms they are used to treat. For example, antibiotics are used aga ...
drugs or immunosuppressive chemicals (as might occur following poison
A poison is any chemical substance that is harmful or lethal to living organisms. 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 figurati ...
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. Po ...
chemotherapy
Chemotherapy (often abbreviated chemo, sometimes CTX and CTx) is the type of cancer treatment that uses one or more anti-cancer drugs (list of chemotherapeutic agents, chemotherapeutic agents or alkylating agents) in a standard chemotherapy re ...
), exposure to ionizing radiation
Ionizing (ionising) radiation, including Radioactive decay, nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionization, ionize atoms or molecules by detaching ...
, or as a result of an infectious disease with immunosuppressive activity (such as with measles
Measles (probably from Middle Dutch or Middle High German ''masel(e)'', meaning "blemish, blood blister") is a highly contagious, Vaccine-preventable diseases, vaccine-preventable infectious disease caused by Measles morbillivirus, measles v ...
, malaria
Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects vertebrates and ''Anopheles'' mosquitoes. Human malaria causes Signs and symptoms, symptoms that typically include fever, Fatigue (medical), fatigue, vomitin ...
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 aburn
A burn is an injury to skin, or other tissues, caused by heat, electricity, chemicals, friction, or ionizing radiation (such as sunburn, caused by ultraviolet radiation). Most burns are due to heat from hot fluids (called scalding), soli ...
or penetrating trauma
Penetrating trauma is an open wound injury that occurs when an object pierces the Human skin, 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-p ...
(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
Injury is physiological damage to the living tissue of any organism, whether in humans, in other animals, or in plants.
Injuries can be caused in many ways, including mechanically with penetration by sharp objects such as teeth or with ...
was the predisposing factor).
= Other types of infection
= Other types of infection consist of mixed,iatrogenic
Iatrogenesis is the causation of a disease, a harmful complication, or other ill effect by any medical activity, including diagnosis, intervention, error, or negligence." Iatrogenic", ''Merriam-Webster.com'', Merriam-Webster, Inc., accessed 27 ...
, 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, fever and anorexia (symptom), decreased appetite. However, approximately 40% of people do not have these t ...
, which is caused by '' Bacteroides fragilis'' and ''Escherichia coli
''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...
''. 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 satisfyKoch's postulates
Koch's postulates ( ) are four criteria designed to establish a causality, causal relationship between a microbe and a disease. The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884, based on earlier concepts described by ...
(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 and anthrax, he i ...
), which require that first, the infectious agent
In biology, a pathogen (, "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.
The term ...
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'') a ...
species cause tuberculosis
Tuberculosis (TB), also known colloquially as the "white death", or historically as consumption, is a contagious disease usually caused by ''Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can al ...
.
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 (, "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 theory of d ...
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 Microaerophile, microaerophilic, Gram-negative bacteria, gram-negative, spirochaete bacterium with subspecies that cause the diseases syphilis, bejel (also known as endemic ...
'', the causative spirochete
A spirochaete () or spirochete is a member of the phylum Spirochaetota (also called Spirochaetes ), which contains distinctive diderm (double-membrane) Gram-negative bacteria, most of which have long, helically coiled (corkscrew-shaped or ...
of syphilis
Syphilis () is a sexually transmitted infection caused by the bacterium ''Treponema pallidum'' subspecies ''pallidum''. The signs and symptoms depend on the stage it presents: primary, secondary, latent syphilis, latent or tertiary. The prim ...
, cannot be cultured ''in vitro'' – however the organism can be cultured in rabbit testes
A testicle or testis ( testes) is the gonad in all male bilaterians, including humans, and is homologous to the ovary in females. Its primary functions are the production of sperm and the secretion of androgens, primarily testosterone.
The ...
. 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 (epidemiology), determinants of health and disease conditions in a defined population, and application of this knowledge to prevent dise ...
, 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 only 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 foun ...
, with a steady level of regular cases occurring in a region; epidemic
An epidemic (from Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of hosts in a given population within a short period of time. For example, in meningococcal infection ...
, 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 a sudden increase in cases and spreads across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. Widespread endemic (epi ...
, 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 calledcontagious disease
A contagious disease is an infectious disease that can be spread rapidly in several ways, including direct contact, indirect contact, and droplet contact.
These diseases are caused by organisms such as parasites, bacteria, fungi, and viruses. ...
s 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 sympto ...
). Thus, a contagious disease is a subset of infectious disease that is especially infective or easily transmitted. All contagious diseases are infectious, but not vice versa. 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 bee ...
) 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 direct 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. The basic reproduction number of an infectious disease measures how easily it spreads through direct or indirect contact.
By anatomic location
Infections can be classified by the anatomic location ororgan system
An organ system is a biological system consisting of a group of organ (biology), organs that work together to perform one or more bodily functions. Each organ has a specialized role in an organism body, and is made up of distinct Tissue (biolog ...
infected, including:
* Urinary tract infection
A urinary tract infection (UTI) is an infection that affects a part of the urinary tract. Lower urinary tract infections may involve the bladder (cystitis) or urethra (urethritis) while upper urinary tract infections affect the kidney (pyel ...
* Skin infection
* Respiratory tract infection
* Odontogenic infection (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, tea ...
or in the closely surrounding tissues)
* Vaginal infections
* Intra-amniotic infection
Chorioamnionitis, also known as amnionitis and 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 Wo ...
In addition, locations of inflammation
Inflammation (from ) is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function (Latin ''calor'', '' ...
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 Cough#Classification, productive or dry cough, ches ...
, 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, intense headache, vomiting and neck stiffness and occasion ...
and salpingitis.
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 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, and other harmful or unwanted substances stuck to th ...
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 external condo ...
, 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 ( and ) is an antimicrobial substance or compound that is applied to living tissue to reduce the possibility of sepsis, infection, or putrefaction. Antiseptics are generally distinguished from ''antibiotics'' by the latter's abili ...
, 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 animal coverings, such as the arthropod exoskeleton, have different ...
* 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 ...
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 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 cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its abili ...
, 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 in ...
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 in ...
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 th ...
( HIV) 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 Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of hosts in a given population within a short period of time. For example, in meningococcal infection ...
.
Another effective way to decrease the transmission rate of infectious diseases is to recognize the effects of small-world networks. 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
Vector most often refers to:
* Euclidean vector, a quantity with a magnitude and a direction
* Disease vector, an agent that carries and transmits an infectious pathogen into another living organism
Vector may also refer to:
Mathematics a ...
-borne pathogens is pest control
Pest control is the regulation or management of a species defined as a pest (organism), pest; such as any animal, plant or fungus that impacts adversely on human activities or environment. The human response depends on the importance of the da ...
.
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
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 dise ...
, such as school closings, lockdown
A lockdown () is a restriction policy for people, community or a country to stay where they are, usually due to specific risks that could possibly harm the people if they move and interact freely.
The term is used for a prison protocol that us ...
s or temporary restrictions (e.g. circuit breakers) 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) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that caus ...
and/or T lymphocytes. Immunity mediated by these two factors may be manifested by:
* a direct effect upon a pathogen, such as antibody-initiated complement-dependent bacteriolysis, opsonoization, phagocytosis
Phagocytosis () is the process by which a cell (biology), 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 ph ...
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 or pyrexia in humans is a symptom of an anti-infection defense mechanism that appears with Human body temperature, body temperature exceeding the normal range caused by an increase in the body's temperature Human body temperature#Fever, s ...
and inflammation
Inflammation (from ) is part of the biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. The five cardinal signs are heat, pain, redness, swelling, and loss of function (Latin ''calor'', '' ...
, 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 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 wat ...
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 1hepatitis C
Hepatitis C is an infectious disease caused by the hepatitis C virus (HCV) that primarily affects the liver; it is a type of viral hepatitis. During the initial infection period, people often have mild or no symptoms. Early symptoms can include ...
treated with Pegylated interferon-alpha-2a or Pegylated interferon-alpha-2b (brand names Pegasys or PEG-Intron) combined with ribavirin, 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
Nature is an inherent character or constitution, particularly of the Ecosphere (planetary), ecosphere or the universe as a whole. In this general sense nature refers to the Scientific law, laws, elements and phenomenon, phenomena of the physic ...
'', 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, 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 ...
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 ...
(including antiprotozoal and antihelminthic) 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
The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...
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 nutr ...
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 antibio ...
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
Macrolides are a class of mostly natural products with a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. Macrolides belong to the polyketide class of natural products. ...
s, quinolones and tetracyclines.
Not all infections require treatment, and for many self-limiting infections the treatment may cause more side-effects than benefits. Antimicrobial stewardship
Antimicrobial stewardship (AMS) refers to coordinated efforts to promote the optimal use of antimicrobial agents, including drug choice, dosing, route, and duration of administration. AMS has been an organized effort of specialists in infectious d ...
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.
Susceptibility to infection
Pandemics such asCOVID-19
Coronavirus disease 2019 (COVID-19) is a contagious disease caused by the coronavirus SARS-CoV-2. In January 2020, the disease spread worldwide, resulting in the COVID-19 pandemic.
The symptoms of COVID‑19 can vary but often include fever ...
show that people dramatically differ in their susceptibility to infection. This may be because of general health, age, or their immune status, e.g. when they have been infected previously. However, it also has become clear that there are genetic factor which determine susceptibility to infection. For instance, up to 40% of SARS-CoV-2
Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is a strain of coronavirus that causes COVID-19, the respiratory illness responsible for the COVID-19 pandemic. The virus previously had the Novel coronavirus, provisional nam ...
infections may be asymptomatic, suggesting that many people are naturally protected from disease. Large genetic studies have defined risk factors for severe SARS-CoV-2 infections, and genome sequences from 659 patients with severe COVID-19 revealed genetic variants that appear to be associated with life-threatening disease. One gene identified in these studies is type I interferon
Interferons (IFNs, ) are a group of signaling proteins made and released by host cells in response to the presence of several viruses. In a typical scenario, a virus-infected cell will release interferons causing nearby cells to heighten ...
(IFN). Autoantibodies against type I IFNs were found in up to 13.7% of patients with life-threatening COVID-19, indicating that a complex interaction between genetics and the immune system
The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
is important for natural resistance to Covid.
Similarly, mutations in the ERAP2 gene, encoding endoplasmic reticulum aminopeptidase 2, seem to increase the susceptibility to the plague, the disease caused by an infection with the bacteria ''Yersinia pestis
''Yersinia pestis'' (''Y. pestis''; formerly ''Pasteurella pestis'') is a Gram-negative bacteria, gram-negative, non-motile bacteria, non-motile, coccobacillus Bacteria, bacterium without Endospore, spores. It is related to pathogens ''Yer ...
''. People who inherited two copies of a complete variant of the gene were twice as likely to have survived the plague as those who inherited two copies of a truncated variant.
Susceptibility also determined the epidemiology of infection, given that different populations have different genetic and environmental conditions that affect infections.
Epidemiology
An estimated 1,680 million people died of infectious diseases in the 20th century and about 10 million in 2010.
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 which coordinates responses to international public health issues and emergencies. It is headquartered in Gen ...
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/AIDS
The HIV, human immunodeficiency virus (HIV) is a retrovirus that attacks the immune system. Without treatment, it can lead to a spectrum of conditions including acquired immunodeficiency syndrome (AIDS). It is a Preventive healthcare, pr ...
, TB and malaria
Malaria is a Mosquito-borne disease, mosquito-borne infectious disease that affects vertebrates and ''Anopheles'' mosquitoes. Human malaria causes Signs and symptoms, symptoms that typically include fever, Fatigue (medical), fatigue, vomitin ...
. 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 ( or ), also known as pertussis or the 100-day cough, is a highly contagious, vaccine-preventable bacterial disease. Initial symptoms are usually similar to those of the common cold with a runny nose, fever, and mild cough, bu ...
, poliomyelitis
Poliomyelitis ( ), commonly shortened to polio, is an infectious disease caused by the poliovirus. Approximately 75% of cases are asymptomatic; mild symptoms which can occur include sore throat and fever; in a proportion of cases more severe ...
, 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, the mortality rate approaches 10%. Signs a ...
, measles
Measles (probably from Middle Dutch or Middle High German ''masel(e)'', meaning "blemish, blood blister") is a highly contagious, Vaccine-preventable diseases, vaccine-preventable infectious disease caused by Measles morbillivirus, measles v ...
and tetanus
Tetanus (), also known as lockjaw, is a bacterial infection caused by ''Clostridium tetani'' and 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 l ...
. 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 is the systematic study of the past, focusing primarily on the Human history, human past. As an academic discipline, it analyses and interprets evidence to construct narratives about what happened and explain why it happened. Some t ...
. A pandemic
A pandemic ( ) is an epidemic of an infectious disease that has a sudden increase in cases and spreads across a large region, for instance multiple continents or worldwide, affecting a substantial number of individuals. Widespread endemic (epi ...
(or global epidemic
An epidemic (from Greek ἐπί ''epi'' "upon or above" and δῆμος ''demos'' "people") is the rapid spread of disease to a large number of hosts in a given population within a short period of time. For example, in meningococcal infection ...
) is a disease that affects people over an extensive geographical area. For example:
* Plague of Justinian
The plague of Justinian or Justinianic plague (AD 541–549) was an epidemic of Plague (disease), plague that afflicted the entire Mediterranean basin, Mediterranean Basin, Europe, and the Near East, especially the Sasanian Empire and the Byza ...
, from 541 to 542, killed between 50% and 60% of Europe's population.
* The Black Death
The Black Death was a bubonic plague pandemic that occurred in Europe from 1346 to 1353. It was one of the list of epidemics, most fatal pandemics in human history; as many as people perished, perhaps 50% of Europe's 14th century population. ...
of 1347 to 1352 killed 25 million in Europe over five 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 (W ...
, 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 exposu ...
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, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...
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 sympto ...
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 (W ...
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), also known colloquially as the "white death", or historically as consumption, is a contagious disease usually caused by ''Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can al ...
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 flu pandemic, also known as the Great Influenza epidemic or by the common misnomer Spanish flu, was an exceptionally deadly global influenza pandemic caused by the H1N1 subtype of the influenza A virus. The earliest docum ...
) 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 sympto ...
kills about 250,000 to 500,000 worldwide each year.
* In 2021, COVID-19 emerged as a major global health crisis, directly causing 8.7 million deaths, making it one of the leading causes of mortality worldwide.
Emerging diseases
In most cases, microorganisms live in harmony with their hosts via mutual orcommensal
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 f ...
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 a ...
between parasite
Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted str ...
and 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 cases, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host. The pathogenicity of an organism—its abili ...
, leading to immunopathological disease.
# Human activity is involved with many emerging infectious disease
An emerging infectious disease (EID) refer to infectious diseases that have either newly appeared in a population or have existed but are rapidly increasing in incidence, geographic range, or severity due to factors such as environmental change ...
s, such as environmental change
Environment most often refers to:
__NOTOC__
* Natural environment, referring respectively to all living and non-living things occurring naturally and the physical and biological factors along with their chemical interactions that affect an organism ...
enabling a parasite to occupy new niches. When that happens, a pathogen
In biology, a pathogen (, "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 theory of d ...
that had been confined to a remote habitat has a wider distribution and possibly a new host organism. 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 virus, bacterium, parasite, fungi, or prion) that can jump from a non-human vertebrate to a human. When h ...
. 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 virus, bacterium, parasite, fungi, or prion) that can jump from a non-human vertebrate to a human. When h ...
human pathogens, including viruses, bacteria, protozoa, and rickettsia, and spread of vector-borne diseases, see also globalization and disease
Globalization, the flow of information, goods, capital, and people across political and geographic boundaries, allows infectious Disease, diseases to rapidly spread around the world, while also allowing the alleviation of factors such as hunger ...
and wildlife disease:
* Encroachment on wildlife habitat
In ecology, habitat refers to the array of resources, 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 manifestation of its ...
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 encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created ...
. 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. 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 in British English) is 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. ...
. 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 tr ...
. 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, theGreek
Greek may refer to:
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 of all kno ...
historian Thucydides
Thucydides ( ; ; 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 been d ...
( – ) was the first person to write, in his account of the plague of Athens
The Plague of Athens (, ) 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 reach. The plague killed an estimated 75, ...
, 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 (; September 129 – AD), often Anglicization, anglicized as Galen () or Galen of Pergamon, was a Ancient Rome, Roman and Greeks, Greek physician, surgeon, and Philosophy, philosopher. Considered to be one o ...
speculated that plagues were spread by "certain seeds of plague", which were present in the air. In the Sushruta Samhita
The ''Sushruta Samhita'' (, ) is an ancient Sanskrit text on medicine and one of the most important such treatises on this subject to survive from the ancient world. The ''Compendium of Sushruta, Suśruta'' is one of the foundational texts of ...
, the ancient Indian physician Sushruta
Suśruta (, ) is the listed author of the '' Suśruta Saṃhiāa'' (''Suśruta's Compendium''), considered to be one of the most important surviving ancient treatises on medicine. It is also considered a foundational text of Ayurveda. The treat ...
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 ( – 22 June 1037), commonly known in the West as Avicenna ( ), was a preeminent philosopher and physician of the Muslim world, flourishing during the Islamic Golden Age, serving in the courts of various Iranian peoples, Iranian ...
(known as Avicenna in Europe) in ''The Canon of Medicine
''The Canon of Medicine'' () is an encyclopedia of medicine in five books compiled by Avicenna (, ibn Sina) and completed in 1025. It is among the most influential works of its time. It presents an overview of the contemporary medical knowle ...
'' (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 Host (biology), hosts in a given population within a short period of time. For example ...
, outlining the classical miasma theory
The miasma theory (also called the miasmic theory) is an abandoned medical theory that held that diseases—such as cholera, chlamydia, or plague—were caused by a ''miasma'' (, Ancient Greek for 'pollution'), a noxious form of "bad air", a ...
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), also known colloquially as the "white death", or historically as consumption, is a contagious disease usually caused by ''Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can al ...
, 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'' ( ; also spelled ''ulema''; ; singular ; feminine singular , plural ) are scholars of Islamic doctrine and law. They are considered the guardians, transmitters, and interpreters of religious knowledge in Islam.
"Ulama ...
in the Ayyubid Sultanate
The Ayyubid dynasty (), also known as the Ayyubid Sultanate, was the founding dynasty of the medieval Sultan of Egypt, Sultanate of Egypt established by Saladin in 1171, following his abolition of the Fatimid Caliphate, Fatimid Caliphate of Egyp ...
who referred to them as '' najasat'' ("impure substances"). The fiqh
''Fiqh'' (; ) is the term for Islamic jurisprudence.Fiqh
Encyclopædia Britannica ''Fiqh'' is of ...
scholar Ibn al-Haj al-Abdari (–1336), while discussing Islamic diet and Encyclopædia Britannica ''Fiqh'' is of ...
hygiene
Hygiene is a set 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 was a bubonic plague pandemic that occurred in Europe from 1346 to 1353. It was one of the list of epidemics, most fatal pandemics in human history; as many as people perished, perhaps 50% of Europe's 14th century population. ...
bubonic plague
Bubonic plague is one of three types of Plague (disease), plague caused by the Bacteria, bacterium ''Yersinia pestis''. One to seven days after exposure to the bacteria, flu-like symptoms develop. These symptoms include fever, headaches, and ...
reached Al-Andalus
Al-Andalus () was the Muslim-ruled area of the Iberian Peninsula. The name refers to the different Muslim states that controlled these territories at various times between 711 and 1492. At its greatest geographical extent, it occupied most o ...
in the 14th century, the Arab physicians Ibn Khatima () and Ibn al-Khatib (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 ( , ) is a Periodization, period of history and a European cultural movement covering the 15th and 16th centuries. It marked the transition from the Middle Ages to modernity and was characterized by an effort to revive and sur ...
, particularly through the writing of the Italian physician Girolamo Fracastoro. 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 art, science and technology. A largely self-taught man in science, he is commonly known as " ...
(1632–1723) advanced the science of microscopy
Microscopy is the technical field of using microscopes to view subjects too small to 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 microscopy: optical mic ...
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 and early germ theory, in part because of hi ...
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, pharmacist, and microbiologist renowned for his discoveries of the principles of vaccination, Fermentation, microbial fermentation, and pasteurization, the la ...
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. It was historically referred to as hydrophobia ("fear of water") because its victims panic when offered liquids to drink. Early symptoms can include fever and abn ...
. 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 and anthrax, he i ...
provided the study of infectious diseases with a scientific basis known as Koch's postulates
Koch's postulates ( ) are four criteria designed to establish a causality, causal relationship between a microbe and a disease. The postulates were formulated by Robert Koch and Friedrich Loeffler in 1884, based on earlier concepts described by ...
. Edward Jenner
Edward Jenner (17 May 1749 – 26 January 1823) was an English 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 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 (W ...
and polio
Poliomyelitis ( ), commonly shortened to polio, is an infectious disease caused by the poliovirus. Approximately 75% of cases are asymptomatic; mild symptoms which can occur include sore throat and fever; in a proportion of cases more severe ...
, 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 wha ...
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 th ...
developed sulphonamides, the first broad spectrum synthetic antibacterial drugs.
Medical specialists
Themedical treatment
A therapy or medical treatment is the attempted remediation of a health problem, usually following a medical diagnosis. Both words, ''treatment'' and ''therapy'', are often abbreviated tx, Tx, or Tx.
As a rule, each therapy has indications an ...
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 transmis ...
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 (epidemiology), determinants of health and disease conditions in a defined population, and application of this knowledge to prevent dise ...
. Generally, infections are initially diagnosed by primary care
Primary care is a model of health care that supports first-contact, accessible, continuous, comprehensive, and coordinated person-focused care. It aims to optimise population health and reduce disparities across the groups by ensuring equitable ...
physicians or internal medicine
Internal medicine, also known as general medicine in Commonwealth nations, is a medical specialty for medical doctors focused on the prevention, diagnosis, and treatment of diseases in adults. Its namesake stems from "treatment of diseases of ...
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 Cough#Classification, productive or dry cough, ches ...
will generally be treated by the internist or the pulmonologist
Pulmonology (, , from Latin ''pulmō, -ōnis'' "lung" and the Greek
Greek may refer to:
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 fam ...
(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 immunocompromise, 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 affe ...
(for example, in AIDS
The HIV, human immunodeficiency virus (HIV) is a retrovirus that attacks the immune system. Without treatment, it can lead to a spectrum of conditions including acquired immunodeficiency syndrome (AIDS). It is a Preventive healthcare, pr ...
or after chemotherapy
Chemotherapy (often abbreviated chemo, sometimes CTX and CTx) is the type of cancer treatment that uses one or more anti-cancer drugs (list of chemotherapeutic agents, chemotherapeutic agents or alkylating agents) in a standard chemotherapy re ...
);
* The infectious agent
In biology, a pathogen (, "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.
The term ...
is of an uncommon nature (e.g. tropical disease
Tropical diseases are diseases that are prevalent in or unique to tropical and subtropical regions. The diseases are less prevalent in temperate climates, due in part to the occurrence of a cold season, which controls the insect population by forc ...
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. Higherfertility rate
The total fertility rate (TFR) of a population is the average number of children that are born to a woman over her lifetime, if they were to experience the exact current age-specific fertility rates (ASFRs) through their lifetime, and they 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 () is a form of polygamy entailing the marriage of a man to several women. The term polygyny is from Neoclassical Greek πολυγυνία (); .
Incidence
Polygyny is more widespread in Africa than in any other continent. Some scholar ...
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'' 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'', ''Allosaurus
''Allosaurus'' ( ) is an extinct genus of theropod dinosaur that lived 155 to 145 million years ago during the Late Jurassic period ( Kimmeridgian to late Tithonian ages). The first fossil remains that could definitively be ascribed to th ...
'', ''Tyrannosaurus
''Tyrannosaurus'' () is a genus of large theropod dinosaur. The type species ''Tyrannosaurus rex'' ( meaning 'king' in Latin), often shortened to ''T. rex'' or colloquially t-rex, is one of the best represented theropods. It lived througho ...
'' 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 2006Space 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. ...
experiment found that '' Salmonella typhimurium'', a bacterium that can cause food poisoning
Foodborne illness (also known as foodborne disease and food poisoning) is any illness resulting from the contamination of food by pathogenic bacteria, viruses, or parasites,
as well as prions (the agents of mad cow disease), and toxins such ...
, became more virulent when cultivated in space
Space is a three-dimensional continuum containing positions and directions. In classical physics, physical space is often conceived in three linear dimensions. Modern physicists usually consider it, with time, to be part of a boundless ...
. On April 29, 2013, scientists in Rensselaer Polytechnic Institute
Rensselaer Polytechnic Institute (; RPI) is a private university, private research university in Troy, New York, United States. It is the oldest technological university in the English-speaking world and the Western Hemisphere. It was establishe ...
, funded by NASA
The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
, reported that, during spaceflight
Spaceflight (or space flight) is an application of astronautics to fly objects, usually spacecraft, into or through outer space, either with or without humans on board. Most spaceflight is uncrewed and conducted mainly with spacecraft such ...
on the International Space Station
The International Space Station (ISS) is a large space station that was Assembly of the International Space Station, assembled and is maintained in low Earth orbit by a collaboration of five space agencies and their contractors: NASA (United ...
, microbes
A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...
seem to adapt to the space environment
Space environment is a branch of astronautics, aerospace engineering and space physics that seeks to understand and address conditions existing in space that affect the design and operation of spacecraft. A related subject, space weather, deals wit ...
in ways "not observed on Earth" and in ways that "can lead to increases in growth and virulence". More recently, in 2017, bacteria
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...
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, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...
s have been observed to survive the vacuum
A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...
of outer space.
See also
*Biological hazard
A biological hazard, or biohazard, is a biological substance that poses a threat (or is a hazard) to the health of living organisms, primarily humans. This could include a sample of a microorganism, virus or toxin that can adversely affect huma ...
* Blood-borne disease
* Coinfection
* Copenhagen Consensus
Copenhagen Consensus is a project that seeks to establish priorities for advancing global welfare using methodologies based on the theory of welfare economics, using cost–benefit analysis. It was conceived and organized around 2004 by Bjørn ...
* Cordon sanitaire
* Epidemiological transition
* Foodborne illness
Foodborne illness (also known as foodborne disease and food poisoning) is any illness resulting from the contamination of food by pathogenic bacteria, viruses, or parasites,
as well as prions (the agents of mad cow disease), and toxins such ...
* 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, healthcare facility. To emphasize both hospital and nonhospital sett ...
* Eradication of infectious diseases
The Extinction, eradication of infectious diseases is the reduction of the prevalence of an Infection, infectious disease in the Earth, global Host (biology), host population to zero.
Two infectious diseases have successfully been eradicated ...
* Infection control
* 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 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 rates. Some causes listed include deaths also included in more specific subordinate causes, and some causes are omitted, ...
* List of diseases caused by insects
* List of infectious diseases
* Mathematical modelling of infectious disease
Mathematical models can project how infectious diseases progress to show the likely outcome of an epidemic (including Plant disease forecasting, in plants) and help inform public health and plant health interventions. Models use basic assumptions ...
* 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). For example, when referring to a group of cells inoculated with virus particles, the MO ...
* Neglected tropical diseases
Neglected tropical diseases (NTDs) are a diverse group of tropical infections that are common in low-income populations in developing regions of Africa, Asia, and the Americas. They are caused by a variety of pathogens, such as viruses, bacteri ...
* Outline of infectious disease concepts
* Sentinel surveillance
* Spillover infection
Spillover infection, also known as pathogen spillover and spillover event, occurs when a reservoir population with a high pathogen prevalence comes into contact with a novel host population. The pathogen is transmitted from the reservoir population ...
* 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 whet ...
* Vaccine-preventable diseases
A vaccine-preventable disease is an infectious disease for which an effective preventive vaccine exists. If a person acquires a vaccine-preventable disease and dies from it, the death is considered a vaccine-preventable death.
The most common and ...
* Waterborne diseases
Waterborne diseases are conditions (meaning adverse effects on human health, such as death, disability, illness or disorders) caused by pathogenic micro-organisms that are transmitted by water. These diseases can be spread while bathing, washing ...
References
External links
European Center for Disease Prevention and Control
U.S. Centers for Disease Control and Prevention
Infectious Disease Society of America
(IDSA)
Vaccine Research Center
Information concerning vaccine research clinical trials for Emerging and re-Emerging Infectious Diseases. *
Microbes & Infection
' (journal) {{DEFAULTSORT:Infection Epidemiology