A term applied to a disease which affects a large number of people in a particular locality at one time. An epidemic disease is usually infectious from person to person, but not necessarily so since many persons in a locality may simply be exposed to the same cause at one time; for example, outbreaks of lead-poisoning are epidemic in this sense.

The conditions which govern the outbreak of epidemics include infected food supplies, such as drinking water contaminated by waste from people with CHOLERA or typhoid fever (see ENTERIC FEVER); milk infected with TUBERCLE bacillus; or ‘fast food’ products contaminated with salmonella. The migrations of certain animals, such as rats, are in some cases responsible for the spread of PLAGUE, from which these animals die in great numbers. Certain epidemics occur at certain seasons: for example, whooping-cough occurs in spring, whereas measles produces two epidemics – as a rule, 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 persons affected and the number who die in successive epidemics: the severity of successive epidemics rises and falls over periods of five or ten years.