An occurrence of a disease or other health condition in a population at a level greater than normally expected for that population, place, and period.
An epidemic occurs when the number of cases rises above the expected baseline. The expected level depends on the disease and setting. For example, a community may normally have only a small number of cases of a particular infection, so a sudden increase above that level could constitute an epidemic.
The term does not require a particular number of cases. An epidemic can occur in a small community or across a large region, depending on what is normally expected there. The term can also be used for conditions other than infectious diseases when they occur at unexpectedly high levels.
QUICK FACT
An epidemic is defined by being above the expected level, not simply by having a large number of cases.
COMMONLY CONFUSED WITH
- Outbreak: often refers to a localized or limited increase in cases. In practice, outbreak and epidemic can overlap, and usage varies by context.
- Pandemic: an epidemic that spreads across multiple countries or continents, generally affecting a large number of people.
- Endemic: a disease or condition that is consistently present at an expected level within a particular population or geographic area.
SEE ALSO