Epidemic, an occurrence of disease that is temporarily of high prevalence. An epidemic occurring over a wide geographical area (e.g., worldwide) is called a pandemic. The rise and decline in epidemic prevalence of an infectious disease is a probability phenomenon dependent upon transfer of an effective dose of the infectious agent from an infected individual to a susceptible one. After an epidemic has subsided, the affected host population contains a sufficiently small proportion of susceptible individuals that reintroduction of the infection will not result in a new epidemic. Since the parasite population cannot reproduce itself in such a host population, the host population as a whole is immune to the epidemic disease, a phenomenon termed herd immunity.
Following an epidemic, however, the host population tends to revert to a condition of susceptibility because of: (1) the deterioration of individual immunity; (2) the removal of immune individuals by death; and (3) the influx of susceptible individuals by birth. Over time the population as a whole again becomes susceptible. The time elapsing between successive epidemic peaks is variable and differs from one disease to another.
By the late 20th century the definition of epidemic had been extended to include outbreaks of any chronic disease or condition (e.g., heart disease or obesity).
The term epidemic is sometimes reserved for disease among human beings; an outbreak of disease among animals other than man is termed epizootic.
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human disease: Classifications of diseasesThe epidemiological classification of disease deals with the incidence, distribution, and control of disorders in a population. To use the example of typhoid, a disease spread through contaminated food and water, it first becomes important to establish that the disease observed is truly caused by
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history of medicine: Immunization against viral diseases…change its character from one epidemic to another. Even so, sufficient progress was made to reduce the chances that a pandemic such as the influenza pandemic of 1918–19, which killed an estimated 25 million people, would occur again. Medical centres were equipped to monitor outbreaks of influenza worldwide in order…
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respiratory disease: Viral infections of the respiratory systemIn epidemics of these diseases, occasional cases occur in which the course is complicated by inflammation of the pericardium—the membrane enclosing the heart—or by a pleural effusion.…
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public health: The Middle AgesDiseases in epidemic proportions included leprosy, bubonic plague, smallpox, tuberculosis, scabies, erysipelas, anthrax, trachoma, sweating sickness, and dancing mania (
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disease: Epidemiology…it is considered to be epidemic during periods of high prevalence. Epidemics prevailing over wide geographic areas are called pandemics.…