Epidemic

An epidemic is an increase in the number of cases of a disease beyond what is normally expected for a particular population, place and season. Epidemiologists judge this against the baseline, or endemic, level: the amount of the disease usually present. The word outbreak means much the same thing but is usually kept for a more local event, such as illness after a shared meal, while an epidemic that spreads across several countries or continents becomes a pandemic.

Epidemics arise when a pathogen, enough susceptible hosts and a route of transmission come together, for example when a new strain appears, a microbe reaches a new setting or people's immunity falls. Investigators classify them by how they spread. In a common-source epidemic, many people are exposed to the same source, such as contaminated food or water. In a propagated epidemic, the infection passes from person to person, so cases appear over several incubation periods.

The label depends on expectations rather than raw numbers. Influenza rises every winter, which is normal, but an unusually severe flu season counts as an epidemic, so the judgment always depends on what is usual for that place.