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, often now simply called "outbreaks" by epidemiologists, 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. 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 Prevention tracks epidemics in the United States.
See also epdemiology.