Table 2. Categories of Infectious Diseases.

Category	Definition
Pandemic	An epidemic that spreads between countries (e.g., Covid-19 was an epidemic when limited to China; with global spread, it became a pandemic)*
Epidemic	Sudden increase in cases above the expected incidence in a region where the disease is not permanently prevalent and is spreading from person to person
Endemic	Infection that is maintained at a baseline level in a geographic area; the infectious agent is always present in the population
Outbreak	Greater-than-anticipated increase in the number of cases of an endemic illness; if not quickly controlled, an outbreak can become an epidemic
Cluster	Increase above the number of expected cases in a place or time period
Sporadic disease	Disease that occurs infrequently and irregularly

^*Covid-19 denotes coronavirus disease 2019.