Defined as an increase, often sudden, in the number of cases of a disease above what is normally expected in a community or region.

The number of cases of a disease that is usually present or expected in a community is referred to as the “baseline” or endemic level of the disease. Some diseases, including rabies and polio, are so rare that a single case warrants epidemiological investigation. Others, such as influenza and malaria, typically have a higher baseline level. Epidemic and outbreak may be used interchangeably, but the latter is often used to refer to a smaller geographical area.

An epidemic is distinguished from a pandemic which is the spread of disease on a much bigger scale with large numbers of people affected globally, as with COVID-19.