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Example of an epidemic showing the number of new infections over time. 2014 West Africa Ebola Epidemic - New Cases per Week.svg
Example of an epidemic showing the number of new infections over time.

An epidemic (from Greek ἐπί epi "upon or above" and δῆμος demos "people") is the rapid spread of disease to a large number of people in a given population within a short period of time. For example, in meningococcal infections, an attack rate in excess of 15 cases per 100,000 people for two consecutive weeks is considered an epidemic. [1] [2]


Epidemics of infectious disease are generally caused by several factors including a change in the ecology of the host population (e.g. increased stress or increase in the density of a vector species), a genetic change in the pathogen reservoir or the introduction of an emerging pathogen to a host population (by movement of pathogen or host). Generally, an epidemic occurs when host immunity to either an established pathogen or newly emerging novel pathogen is suddenly reduced below that found in the endemic equilibrium and the transmission threshold is exceeded. [3]

An epidemic may be restricted to one location; however, if it spreads to other countries or continents and affects a substantial number of people, it may be termed a pandemic. [1] The declaration of an epidemic usually requires a good understanding of a baseline rate of incidence; epidemics for certain diseases, such as influenza, are defined as reaching some defined increase in incidence above this baseline. [2] A few cases of a very rare disease may be classified as an epidemic, while many cases of a common disease (such as the common cold) would not.


The term epidemic derives from a word form attributed to Homer's Odyssey , which later took its medical meaning from the Epidemics, a treatise by Hippocrates. [4] Before Hippocrates, epidemios, epidemeo, epidamos, and other variants had meanings similar to the current definitions of "indigenous" or "endemic". [4] Thucydides' description of the Plague of Athens is considered one of the earliest accounts of a disease epidemic. [4] By the early 17th century, the terms endemic and epidemic referred to contrasting conditions of population-level disease, with the endemic condition at low rates of occurrence and the epidemic condition widespread. [5] The term "epidemic" has become emotionally charged. [6]

The Atlanta Center for Disease Control defines epidemic broadly: "Epidemic: the occurrence of more cases of disease, injury, or other health condition than expected in a given area or among a specific group of persons during a particular period. Usually, the cases are presumed to have a common cause or to be related to one another in some way (see also outbreak)." [1] The terms "epidemic" and "outbreak" have often been used interchangeably. Researchers Manfred S. Green and colleagues propose that the latter term be restricted to smaller events, pointing out that Chambers Concise Dictionary and Stedman's Medical Dictionary acknowledge this distinction. [6]


The Plague of Athens (c. 1652-1654) by Michiel Sweerts, illustrating the devastating epidemic that struck Athens in 430 BC, as described by the historian Thucydides Plague in an Ancient City LACMA AC1997.10.1 (1 of 2).jpg
The Plague of Athens (c. 1652–1654) by Michiel Sweerts, illustrating the devastating epidemic that struck Athens in 430 BC, as described by the historian Thucydides

There are several changes that may occur in an infectious agent that may trigger an epidemic. These include: [1] :55

An epidemic disease is not required to be contagious, [2] [4] and the term has been applied to West Nile fever [2] and the obesity epidemic (e.g. by the World Health Organisation [7] ), among others. [4]

The conditions which govern the outbreak of epidemics include infected food supplies such as contaminated drinking water and the migration of populations of certain animals, such as rats or mosquitoes, which can act as disease vectors.

Certain epidemics occur at certain seasons. For example, whooping-cough occurs in spring, whereas measles produces two epidemics, one in winter and one in March. Influenza, the common cold, and other infections of the upper respiratory tract, such as sore throat, occur predominantly in the winter. There is another variation, both as regards the number of people affected and the number who die in successive epidemics: the severity of successive epidemics rises and falls over periods of five or ten years. [8]


Common source outbreak

In a common source outbreak epidemic, the affected individuals had an exposure to a common agent. If the exposure is singular and all of the affected individuals develop the disease over a single exposure and incubation course, it can be termed a point source outbreak. If the exposure was continuous or variable, it can be termed a continuous outbreak or intermittent outbreak, respectively. [1] :56

Propagated outbreak

In a propagated outbreak, the disease spreads person-to-person. Affected individuals may become independent reservoirs leading to further exposures. [1] :56

Many epidemics will have characteristics of both common source and propagated outbreaks (sometimes referred to as mixed outbreak).

For example, secondary person-to-person spread may occur after a common source exposure or an environmental vectors may spread a zoonotic diseases agent. [1] :56–58



Preparations for an epidemic include having a disease surveillance system; the ability to quickly dispatch emergency workers, especially local-based emergency workers; and a legitimate way to guarantee the safety and health of health workers. [10] [11]

Effective preparations for a response to a pandemic are multi-layered. The first layer is a disease surveillance system. Tanzania, for example, runs a national lab that runs testing for 200 health sites and tracks the spread of infectious diseases. The next layer is the actual response to an emergency. According to U.S.-based columnist Michael Gerson in 2015, only the U.S. military and NATO have the global capability to respond to such an emergency. [10]

See also

Related Research Articles

Zoonosis disease that can be transmitted from one species to another

A zoonosis is an infectious disease caused by bacteria, viruses, or parasites that spread from non-human animals to humans.

Infection Invasion of an organisms body tissues by disease-causing agents

Infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to the infectious agents and the toxins they produce. Infectious disease, also known as transmissible disease or communicable disease, is illness resulting from an infection.

Tularemia primary bacterial infectious disease that has material basis in Francisella tularensis, which is transmitted by dog tick bite (Dermacentor variabilis), transmitted by deer flies (Chrysops sp) or transmitted by contact with infected animal tissues.

Tularemia, also known as rabbit fever, is an infectious disease caused by the bacterium Francisella tularensis. Symptoms may include fever, skin ulcers, and enlarged lymph nodes. Occasionally, a form that results in pneumonia or a throat infection may occur.

Disease outbreak Sudden increase in occurrences of a disease

In epidemiology, an outbreak is a sudden increase in occurrences of a disease in a particular time and place. It may affect a small and localized group or impact upon thousands of people across an entire continent. Four linked cases of a rare infectious disease may be sufficient to constitute an outbreak. Outbreaks include epidemics, which term is normally only used for infectious diseases, as well as diseases with an environmental origin, such as a water or foodborne disease. They may affect a region in a country or a group of countries. Pandemics are near-global disease outbreaks when multiple countries across the world are infected.

Asymptomatic carrier a person or other organism that has become infected with a pathogen, without displaying signs or symptoms

An asymptomatic carrier is a person or other organism that has become infected with a pathogen, but that displays no signs or symptoms.

In medicine, public health, and biology, transmission is the passing of a pathogen causing communicable disease from an infected host individual or group to a particular individual or group, regardless of whether the other individual was previously infected.

In epidemiology, an infection is said to be endemic in a population when that infection is constantly maintained at a baseline level in a geographic area without external inputs. For example, chickenpox is endemic in the UK, but malaria is not. Every year, there are a few cases of malaria reported in the UK, but these do not lead to sustained transmission in the population due to the lack of a suitable vector. While it might be common to say that AIDS is "endemic" in many African countries, meaning found in an area, this is a use of the word in its etymological, rather than epidemiological, form. AIDS cases in Africa are increasing, so the disease is not in an endemic steady state. The spread of AIDS in Africa could be correctly called an epidemic, however.

Natural reservoir a living host, such as an animal or a plant, inside of which an infectious pathogen naturally lives and reproduces

In infectious disease ecology and epidemiology, a natural reservoir, also known as a disease reservoir or a reservoir of infection, is the population of organisms or the specific environment in which an infectious pathogen naturally lives and reproduces, or upon which the pathogen primarily depends for its survival. A reservoir is usually a living host of a certain species, such as an animal or a plant, inside of which a pathogen survives, often without causing disease for the reservoir itself. By some definitions a reservoir may also be an environment external to an organism, such as a volume of contaminated air or water.

Swine influenza Infection caused by influenza viruses endemic to pigs

Swine influenza is an infection caused by any one of several types of swine influenza viruses. Swine influenza virus (SIV) or swine-origin influenza virus (S-OIV) is any strain of the influenza family of viruses that is endemic in pigs. As of 2009, the known SIV strains include influenza C and the subtypes of influenza A known as H1N1, H1N2, H2N1, H3N1, H3N2, and H2N3.

Influenza A virus subtype H1N1 Subtype of the influenza A virus

Influenza A virus subtype H1N1 (A/H1N1) is the subtype of influenza A virus that was the most common cause of human influenza (flu) in 2009, and is associated with the 1918 outbreak known as the Spanish flu.

Infection control is the discipline concerned with preventing nosocomial or healthcare-associated infection, a practical sub-discipline of epidemiology. It is an essential part of the infrastructure of health care. Infection control and hospital epidemiology are akin to public health practice, practiced within the confines of a particular health-care delivery system rather than directed at society as a whole. Anti-infective agents include antibiotics, antibacterials, antifungals, antivirals and antiprotozoals.

Transmission and infection of H5N1

Transmission and infection of H5N1 from infected avian sources to humans has been a concern since the first documented case of human infection in 1997, due to the global spread of H5N1 that constitutes a pandemic threat.

Isolation (health care) various measures taken to prevent contagious diseases from being spread among patients, health care workers, and visitors

In health care facilities, isolation represents one of several measures that can be taken to implement infection control: the prevention of contagious diseases from being spread from a patient to other patients, health care workers, and visitors, or from outsiders to a particular patient. Various forms of isolation exist, in some of which contact procedures are modified, and others in which the patient is kept away from all others. In a system devised, and periodically revised, by the U.S. Centers for Disease Control and Prevention (CDC), various levels of patient isolation comprise application of one or more formally described "precaution".

Influenza Infectious disease, "the flu"

Influenza, commonly known as "the flu", is an infectious disease caused by an influenza virus. Symptoms can be mild to severe. The most common symptoms include: high fever, runny nose, sore throat, muscle and joint pain, headache, coughing, and feeling tired. These symptoms typically begin two days after exposure to the virus and most last less than a week. The cough, however, may last for more than two weeks. In children, there may be diarrhea and vomiting, but these are not common in adults. Diarrhea and vomiting occur more commonly in gastroenteritis, which is an unrelated disease and sometimes inaccurately referred to as "stomach flu" or the "24-hour flu". Complications of influenza may include viral pneumonia, secondary bacterial pneumonia, sinus infections, and worsening of previous health problems such as asthma or heart failure.

Social distancing Infection control technique

Social distancing, or physical distancing, is a set of infection control actions intended to stop or slow down the spread of a contagious disease. The objective of social distancing is to reduce the probability of contact between persons carrying an infection, and others who are not infected, so as to minimize disease transmission, morbidity and ultimately mortality.

Airborne disease disease that is caused by pathogens and transmitted through the air

An airborne disease is any disease that is caused by pathogens that can be transmitted through the air by both small, dry particles, and as larger liquid droplets. Such diseases include many of considerable importance both in human and veterinary medicine. The relevant pathogens may be viruses, bacteria, or fungi, and they may be spread through breathing, talking, coughing, sneezing, raising of dust, spraying of liquids, toilet flushing or any activities which generate aerosol particles or droplets. Human airborne diseases do not include conditions caused by air pollution such as Volatile Organic Compounds (VOCs), gases and any airborne particles.

Influenza prevention involves taking steps that one can use to decrease their chances of contracting flu viruses, such as the Pandemic H1N1/09 virus, responsible for the 2009 flu pandemic.

Transmission-based precautions are additional infection-control precautions in health care, and the latest routine infection prevention and control practices applied for patients who are known or suspected to be infected or colonized with infectious agents, including certain epidemiologically important pathogens. The latter require additional control measures to effectively prevent transmission.

Super-spreader a host of a disease that disproportionally infects more secondary contacts than others infected with the same disease

A super-spreader is an unusually contagious organism infected with a disease. In context of a human-borne illness, a super-spreader is an individual who is more likely to infect others, compared with a typical infected person. Such super-spreaders are of particular concern in epidemiology.

Ira Longini is an American biostatistician and infectious disease epidemiologist.


  1. 1 2 3 4 5 6 7 Principles of Epidemiology, Third Edition (PDF). Atlanta, Georgia: Centers for Disease Control and Prevention. 2012.
  2. 1 2 3 4 Green MS; Swartz T; Mayshar E; Lev B; Leventhal A; Slater PE; Shemer Js (January 2002). "When is an epidemic an epidemic?". Isr. Med. Assoc. J. 4 (1): 3–6. PMID   11802306.
  3. "epidemic". The Encyclopedia of Ecology and Environmental Management, Blackwell Science. Oxford: Blackwell Publishers, 1998. Credo Reference. Web. 17 September 2012.
  4. 1 2 3 4 5 Martin PM, Martin-Granel E (June 2006). "2,500-year evolution of the term epidemic" (PDF). Emerging Infect. Dis. 12 (6): 976–80. doi:10.3201/eid1206.051263. PMC   3373038 . PMID   16707055.
  5. Lodge, T. (1603). A treatise of the plague: containing the nature, signes, and accidents of the same, with the certaine and absolute cure of the fevers, botches and carbuncles that raigne in these times. London: Edward White.
  6. 1 2 Green MS, Swartz T, Mayshar E, Lev B, Leventhal A, Slater PE, Shemer J (2002). "When is an epidemic an epidemic?" (PDF). Israel Medical Association Journal. 4 (1): 3–6. PMID   11802306.
  7. Controlling the global obesity epidemic, the World Health Organisation
  8. "Epidemic." Black's Medical Dictionary, 42nd Edition. London: A&C Black, 201e. Web. 17 September 2012.
  9. "transmission." Saunders Comprehensive Veterinary Dictionary. Philadelphia: Elsevier Health Sciences, 2007. Credo Reference. Web. 17 September 2012.
  10. 1 2 Gerson, Michael, "The next epidemic". The Washington Post , 26 March 2015.
  11. Gates, Bill, "The Next Epidemic — Lessons from Ebola". The New England Journal of Medicine , April 9, 2015.

Further reading

External video
Nuvola apps kaboodle.svg Presentation by Brown on Influenza, March 5, 2019, C-SPAN