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epidemic, outbreak of disease that affects a much greater number of people than is usual for the locality or that spreads to regions where it is ordinarily not present. A disease that tends to be restricted to a particular region (endemic disease) can become epidemic if nonimmune persons are present in large numbers (as in time of war or during pilgrimages), if the infectious agent is more virulent than usual, or if distribution of the disease is more easily effected. Cholera and plague, endemic in parts of Asia, can become epidemic under the above conditions, as can dysentery and many other infections. Epidemics, often now simply called “outbreaks” by epidemiologists, may also be caused by new disease agents in the human population, such as the Ebola virus. A worldwide epidemic is known as a pandemic, e.g., the influenza pandemic of 1918 or the AIDS pandemic beginning in the 1980s. Officially, the World Health Organization considers any disease outbreak that is spreading unchecked in two different regions of the worlds to be a pandemic; classification as a pandemic is not an indicator of the severity of a disease. A disease is said to be sporadic when only a few cases occur here and there in a given region. Epidemic disease is controlled by various measures, depending on whether transmission is through respiratory droplets, food and water contaminated with intestinal wastes, insect vectors, or other means. The Centers for Disease Control and Prevention tracks epidemics in the United States.

See also epdemiology.

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The occurrence of cases of disease in excess of what is usually expected for a given period of time. Epidemics are commonly thought to involve outbreaks of acute infectious disease, such as measles, polio, or streptococcal sore throat. More recently, other types of health-related events such as homicide, drownings, and even hysteria have been considered to occur as “epidemics.”

Confusion sometimes arises because of overlap between the terms epidemic, outbreak, and cluster. Although they are closely related, epidemic may be used to suggest problems that are geographically widespread, while outbreak and cluster are reserved for problems that involve smaller numbers of people or are more sharply defined in terms of the area of occurrence. For example, an epidemic of influenza could involve an entire state or region, whereas an outbreak of gastroenteritis might be restricted to a nursing home, school, or day-care center. The term cluster may be used to refer to noncommunicable disease states.

In contrast to epidemics, endemic problems are distinguished by their consistently high levels over a long period of time. Lung cancer in males has been endemic in the United States, whereas the surge of lung cancer cases in women in the United States represents an epidemic problem that has resulted from increase in cigarette smoking among women in general. A pandemic is closely related to an epidemic, but it is a problem that has spread over a considerably larger geographic area; influenza pandemics are often global.

Disease and epidemics occur as a result of the interaction of three factors, agent, host, and environment. Agents cause the disease, hosts are susceptible to it, and environmental conditions permit host exposure to the agent. An understanding of the interaction between agent, host, and environment is crucial for the selection of the best approach to prevent or control the continuing spread of an epidemic.

For infectious diseases, epidemics can occur when large numbers of susceptible persons are exposed to infectious agents in settings or under circumstances that permit the spread of the agent. Spread of an infectious disease depends primarily on the chain of transmission of an agent: a source of the agent, a route of exit from the host, a suitable mode of transmission between the susceptible host and the source, and a route of entry into another susceptible host. Modes of spread may involve direct physical contact between the infected host and the new host, or airborne spread, such as coughing or sneezing. Indirect transmission takes place through vehicles such as contaminated water, food, or intravenous fluids; inanimate objects such as bedding, clothes, or surgical instruments; or a biological vector such as a mosquito or flea. See Epidemiology, Infectious disease

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



the spread of an infectious human disease that substantially exceeds the ordinary (sporadic) sick rate in a particular area.

Epidemics are caused by social and biological factors. Their basis is the epidemic process, that is, the continuous transmission of the causative agent of the infection and an unbroken chain of successively developing and interdependent infectious conditions (disease, bacteria carrier state) in a group of people. The spread of a disease is sometimes pandemic in nature; in other words, under certain natural or sociohygienic conditions, a comparatively high incidence of a disease may be recorded in a particular area over a long period of time.

The origin and course of an epidemic are influenced both by processes occurring under natural conditions (natural focality, epizootic) and, especially, by social factors (public utilities, living conditions, quality of health care). Depending on the nature of the disease, the main routes by which an infection spreads during an epidemic may be water and food, as in dysentery and typhoid; airborne droplets, as in influenza; and person-to-person transmission, as in malaria and typhus. Often the causative agent is transmitted by several means.

The study of epidemics and methods of controlling them is called epidemiology.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A sudden increase in the incidence rate of a disease to a value above normal, affecting large numbers of people and spread over a wide area.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


1. (esp of a disease) attacking or affecting many persons simultaneously in a community or area
2. a widespread occurrence of a disease
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
Many health care providers remain fearful of treating these patients and echo the behaviors seen during the early days (and in some cases today) in our own epidemic.
To anticipate epidemics, scientists with the International Medical Research Center of Franceville (CIRMF) in Gabon teamed with several academic institutions, government ministries, and international conservation groups.
* The second epidemic in 1957 (Asian flu) left 70,000 dead in the U.S.
Experts are predicting that the coming epidemic will be particularly nasty, as the notification aim hospital admission rates have not returned to the levels before the last epidemic.
Working primarily in northern Italy, England, and France, reading scores of chronicles and plague tracts, and compiling statistics from thousands of burial records, obituaries, and wills, Cohn finds evidence of epidemics that do not match modern plague's contagion, speed of transmission, seasonality, social characteristics of its victims, mortality rates, or the massive death of rats.
In these, the author has traveled to the site of the epidemic out-break and interviewed in person the common folk who were there at the beginning.
Yet the dramaturgy of death depicted by Stannard highlights how generational relations hinged on norms of emotional avoidance and beliefs in evil pollution, crucial undercurrents affecting Zulu responses to the 1890s rinderpest epizootic, the first of three lethal epidemics sketched below.
The balance of the book takes these ideas and applies them to puzzling situations and epidemics in motion and at rest on the planet.
Experts who develop mathematical models of STD epidemics call these people "core transmitters." They are, as one writer put it, the people who "keep the critical mass critical." If an infected core transmitter comes into contact with men who are unlikely to spread the disease--even a large number of them--he will infect many, but won't trigger an epidemic.
and other rich countries to control these three epidemics worldwide would be about the cost of a movie and a bag of popcorn for each person once a year.
In more modern times, the British used germ warfare in the French and Indian war, with devastating results: Suspecting that certain Indian tribes were secretly in cahoots with the French, they gifted to the Indians blankets contaminated with smallpox, leading to epidemics that killed hundreds of Indians and virtually wiped out entire tribes.
Three million acres were infested during the 1990s alone, and the epidemic reached its peak of one million in 1996.