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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. Choleracholera
or Asiatic cholera,
acute infectious disease caused by strains of the bacterium Vibrio cholerae that have been infected by bacteriophages. The bacteria, which are found in fecal-contaminated food and water and in raw or undercooked seafood, produce a
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 and plagueplague,
any contagious, malignant, epidemic disease, in particular the bubonic plague and the black plague (or Black Death), both forms of the same infection. These acute febrile diseases are caused by Yersinia pestis (Pasteurella pestis
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, endemic in parts of Asia, can become epidemic under the above conditions, as can dysenterydysentery
, inflammation of the intestine characterized by the frequent passage of feces, usually with blood and mucus. The two most common causes of dysentery are infection with a bacillus (see bacteria) of the Shigella group, and infestation by an ameba,
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 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 virusEbola virus
, a member of a family (Filovidae) of RNA viruses that cause hemorrhagic fevers. The virus, named for the region in Congo (Kinshasa) where it was first identified in 1976, emerged from the rain forest, where it survives in as yet unconfirmed hosts, possibly
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. A worldwide epidemic is known as a pandemic, e.g., the influenzainfluenza
or flu,
acute, highly contagious disease caused by a RNA virus (family Orthomyxoviridae); formerly known as the grippe. There are three types of the virus, designated A, B, and C, but only types A and B cause more serious contagious infections.
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 pandemic of 1918 or the AIDSAIDS
or acquired immunodeficiency syndrome,
fatal disease caused by a rapidly mutating retrovirus that attacks the immune system and leaves the victim vulnerable to infections, malignancies, and neurological disorders. It was first recognized as a disease in 1981.
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 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 PreventionCenters for Disease Control and Prevention
(CDC), agency of the U.S. Public Health Service since 1973, with headquarters in Atlanta; it was established in 1946 as the Communicable Disease Center.
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 tracks epidemics in the United States.

See also epdemiologyepidemiology,
field of medicine concerned with the study of epidemics, outbreaks of disease that affect large numbers of people. Epidemiologists, using sophisticated statistical analyses, field investigations, and complex laboratory techniques, investigate the cause of a
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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



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.


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.


1. (esp of a disease) attacking or affecting many persons simultaneously in a community or area
2. a widespread occurrence of a disease