An epidemic is the occurrence of more cases of a disease than would be expected in a limited community or region during a given time period. Whenever a transmission can be restricted to one community or region, which means that many people in one community or region catch an infectious disease, experts talk about an epidemic.

In Germany experts talk about an epidemic when about 10 to 20 percent of all people get infected during the yearly flu wave. Normally there are a lot less infections.