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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 infectious disease to a large number of people in a given population within a short period of time, usually two weeks or less. 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]

Ancient Greek Version of the Greek language used from roughly the 9th century BCE to the 6th century CE

The Ancient Greek language includes the forms of Greek used in Ancient Greece and the ancient world from around the 9th century BCE to the 6th century CE. It is often roughly divided into the Archaic period, Classical period, and Hellenistic period. It is antedated in the second millennium BCE by Mycenaean Greek and succeeded by medieval Greek.

In epidemiology, the attack rate is the biostatistical measure of frequency of morbidity, or speed of spread, in an at risk population. It is used in hypothetical predictions and during actual outbreaks of disease. An at risk population is defined as one that has no immunity to the attacking pathogen which can be either a novel pathogen or an established pathogen. It is used to project the number of victims to expect during an epidemic. This aids in marshalling resources for delivery of medical care as well as production of vaccines and/or anti-viral and anti-bacterial medicines. The rate is arrived at by taking the number of new cases in the population at risk and dividing by the number of persons at risk in the population.


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.

Pandemic global epidemic of infectious disease

A pandemic is an epidemic of disease that has spread across a large region; for instance multiple continents, or even worldwide. This may include communicable and noncommunicable diseases.

A baseline in medicine is information found at the beginning of a study or other initial known value which is used for comparison with later data. The concept of a baseline is essential to the daily practice of medicine in order to establish a relative rather than absolute meaning to data. The meaning of baseline in medicine is very similar to that of the running baseline (baseball) being the direct path that a baserunner is taking to the base he is in route to. If the baserunner is outside the 3 foot margin around his running baseline, then he is considered out. Whereas if a patient with kidney failure suddenly has a creatinine of 5.0 mg/dL, then his creatinine is out of his normal. For that person with kidney failure, absolute normal no longer applies because he will never again be able to obtain an absolutely normal creatinine level with kidneys that no longer function properly.

Incidence (epidemiology) measure of the probability of occurrence of a given medical condition in a population within a specified period of time

Incidence in epidemiology is a measure of the probability of occurrence of a given medical condition in a population within a specified period of time. Although sometimes loosely expressed simply as the number of new cases during some time period, it is better expressed as a proportion or a rate with a denominator.


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

Virulence is a pathogen's or microbe's ability to infect or damage a host.

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 [5] ), among others. [4]

A contagious disease is a subset category of transmissible diseases, which are transmitted to other persons, either by physical contact with the person suffering the disease, or by casual contact with their secretions or objects touched by them or airborne route among other routes.

West Nile fever human disease

West Nile fever is a viral infection typically spread by mosquitoes. In about 80% of infections people have few or no symptoms. About 20% of people develop a fever, headache, vomiting, or a rash. In less than 1% of people, encephalitis or meningitis occurs, with associated neck stiffness, confusion, or seizures. Recovery may take weeks to months. The risk of death among those in whom the nervous system is affected is about 10%.

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. [6]


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 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




Experts[ who? ] suggest that the best way to prepare for an epidemic is to have a disease surveillance system, be able to quickly dispatch emergency workers, especially local-based emergency workers, and have a legitimate way to guarantee the safety and health of health workers. [8]

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, only the U.S. military and NATO have the global capability to respond to such an emergency. [8]

Gates proposed that the world responded slowly to the Ebola virus outbreak because of a lack of preparation. Two weeks after the 2013 typhoon hit the Philippines, over 150 foreign medical teams were on the ground helping with injured victims. After the 2005 earthquake in Pakistan, a team of aid workers who had been helping Afghan refugees for several decades were able to get to the victims in less than 24 hours. Dr. Bruce Aylward, assistant director general for emergencies at the World Health Organization, says that in the case of the Ebola outbreak, "there was no way anyone could guarantee the right of medical evacuation for people affected by Ebola.”

The CDC will scale back global disease prevention efforts by 80 percent by October 2019 due to a lack of funding (as funding had been temporarily increased in 2014 to address the Ebola epidemic). [9]


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. [10]

See also

Related Research Articles

Cholera Bacterial infection of the small intestine

Cholera is an infection of the small intestine by some strains of the bacterium Vibrio cholerae. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea that lasts a few days. Vomiting and muscle cramps may also occur. Diarrhea can be so severe that it leads within hours to severe dehydration and electrolyte imbalance. This may result in sunken eyes, cold skin, decreased skin elasticity, and wrinkling of the hands and feet. Dehydration can cause the skin to turn bluish. Symptoms start two hours to five days after exposure.

Zoonosis infectious disease that is transmitted between species (sometimes by a vector) from animals other than humans to humans or from humans to other animals

Zoonoses are infectious diseases that can be naturally transmitted between animals and humans.

Infection invasion of a host by disease-causing organisms

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.

Shigellosis Human disease

Shigellosis is an infection of the intestines caused by Shigella bacteria. Symptoms generally start one to two days after exposure and include diarrhea, fever, abdominal pain, and feeling the need to pass stools even when the bowels are empty. The diarrhea may be bloody. Symptoms typically last five to seven days. Complications can include reactive arthritis, sepsis, seizures, and hemolytic uremic syndrome.

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.

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. Two 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.

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, the basic reproduction number of an infection can be thought of as the number of cases one case generates on average over the course of its infectious period, in an otherwise uninfected population.

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 Africa, 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. It is correct to call the spread of AIDS in Africa an epidemic.

Mathematical models can project how infectious diseases progress to show the likely outcome of an epidemic and help inform public health interventions. Models use some basic assumptions and mathematics to find parameters for various infectious diseases and use those parameters to calculate the effects of different interventions, like mass vaccination programmes. The modelling can help in deciding which intervention/s to avoid and which to trial.

Natural reservoir long-term host of the pathogen of an infectious disease

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.

Infection control is the discipline concerned with preventing nosocomial or healthcare-associated infection, a practical sub-discipline of epidemiology. It is an essential, though often underrecognized and undersupported, 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.

Fatty has a big fatness disease. This is the cause of globalisation fatness.

Bubonic plague Human and animal disease

Bubonic plague is one of three types of plague caused by bacterium Yersinia pestis. One to seven days after exposure to the bacteria, flu-like symptoms develop. These symptoms include fever, headaches, and vomiting. Swollen and painful lymph nodes occur in the area closest to where the bacteria entered the skin. Occasionally, the swollen lymph nodes may break open.

Isolation (health care) various measures taken to prevent contagious diseases from being spread

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".

Social distancing

Social distancing is a term applied to certain nonpharmaceutical infection control actions that are taken by public health officials to stop or slow down the spread of a highly 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. 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 generates 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, though their study and prevention may help inform the science of airborne disease transmission.

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.


A super-spreader is a host—an organism infected with a disease—that infects, disproportionately, more secondary contacts than other hosts who are, also, infected with the same disease. A sick human can be a super-spreader; they would be more likely to infect others than most people with the disease. Super-spreaders are thus of high concern in epidemiology.

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 infection is transmitted from the reservoir population and may or may not be transmitted within the host population.


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  8. 1 2 Gerson, Michael (30 March 2015). "The next epidemic". The Greenville News. South Carolina, US.
  9. Thomsen, Jacqueline (2 February 2018). "CDC to cut global disease prevention efforts by 80 percent".
  10. 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.