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 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. 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.

or Spanish influenza epidemic

Most severe influenza outbreak of the 20th century. It apparently started as a fairly mild strain in a U.S. army camp in early March 1918. Troops sent to fight in World War I spread the virus to western Europe. Outbreaks occurred in nearly every inhabited part of the world, spreading from ports to cities along transportation routes. Pneumonia often developed quickly and killed within two days. Among the most deadly epidemics in history, it left an estimated 25 million dead; unusually, half the deaths were among 20- to 40-year-olds.

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or epidemic parotitis

Acute contagious viral disease with inflammatory swelling of the salivary glands. Epidemics often occur, mostly among 5- to 15-year-olds. Cold symptoms with low fever are followed by swelling and stiffening in front of the ear, often on both sides. This rapidly spreads toward the neck and under the jaw. Pain is seldom severe, with little redness, but chewing and swallowing are difficult. During recovery in patients past puberty, other glands may be affected, but usually not seriously. The testes may atrophy, but sterility is very rare. While inflammation of the brain and meninges is fairly common, chances of recovery are good. Mumps needs no special treatment, and patients usually develop immunity. Vaccination can prevent it.

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