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. A community may have an outbreak of the flu or measles as that specific organism spreads through the water or food in that area.

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.