An epidemic refers to a disease outbreak in a community, population, or region that affects a larger number of individuals than what is typically expected. Examples of epidemics include smallpox, measles, and polio.
Epidemics, however, are not exclusively contagious diseases. Other health-related diseases, like increasing obesity rates, or rates of individuals who smoke may also be considered epidemics since the occurrence rates are higher than what one may expect for general populations.
There are two classifications of outbreaks: propagated and common-source.
- A propagated outbreak occurs when a disease is passed between people through direct contact or through sharing an item that carries the disease. Sexually transmitted diseases are a common example of this type of outbreak.
- A common-source outbreak occurs when a group of people contracts a disease from being exposed the same infectious agent. Food sickness from eating the same food is a common example of this type of outbreak.
There are a variety of causes of epidemics, such as changes in weather, introducing a disease to a population that does not have a tolerance to it, an increase in the potential virulence of a disease, or other changes to the location or demographic which allow a disease to spread more easily.