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epidemic ep·i·dem·ic (ěp'ĭ-děm'ĭk) or ep·i·dem·i·cal (-ĭ-kəl)
Spreading rapidly and extensively by infection and affecting many individuals in an area or a population at the same time, as of a disease or illness. n.
An outbreak or unusually high occurrence of a disease or illness in a population or area.
An outbreak of a disease or illness that spreads rapidly among individuals in an area or population at the same time. See also endemic, pandemic.
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.