An outbreak of an illness occurs when there is an increase in the number of people who live in the same area and are infected around the same time by an organism such as a virus or bacteria. Some examples include COVID-19, flu, or measles.

Organisms can spread through the air, water, or food. Specific organisms passed to people from insects or rodents can cause outbreaks too. These outbreaks usually do not spread much beyond the region where the rodents or insects live.