Disease outbreaks

A disease outbreak is the occurrence of disease cases in excess of normal expectancy. The number of cases varies according to the disease-causing agent, and the size and type of previous and existing exposure to the agent.

Disease outbreaks are usually caused by an infection, transmitted through person-to-person contact, animal-to-person contact, or from the environment or other media.

Outbreaks may also occur following exposure to chemicals or to radioactive materials. For example, Minamata disease is caused by exposure to mercury.

Occasionally the cause of an outbreak is unknown, even after thorough investigation.