Throughout history epidemics of diseases like cholera, smallpox, influenza, SARS and foodborne illnesses have affected human populations. It is essential that such outbreaks are identified and controlled in order to prevent the further spread of disease. This is the task of outbreak investigation, a subfield of epidemiology. The methods of descriptive and, where appropriate, analytical epidemiology are employed in outbreak investigations. The work of Edward Jenner who used a preventive vaccination to curb an epidemic of smallpox in England in the 18th century and John Snow who discovered that contaminated water was the cause of an epidemic of cholera in London in the 19th century laid the foundation for modern outbreak investigations.

In general, an outbreak is defined as a greater than expected number of cases of a disease in a certain area at a specific time. The number of cases can be compared to a normal number of cases based on historical information from health department surveillance data, disease registries or hospital discharge records. However, the difference between actual and expected numbers of cases may not indicate a true outbreak. Variations in reporting procedures and sudden changes in population size can influence these numbers.

Successful outbreak investigations can deliver valuable information such as the identification of a new mode of transmission of an infectious agent. For example, outbreak investigations led to the discovery that E. coli O157:H7 infection could be transmitted by eating undercooked hamburger meat (Center for Disease Control and Prevention 1993). Outbreaks can also be continuous or intermittent, with or without a point source. For example, the cholera epidemic that sickened many in Broad Street in London in 1854 was traced to a contaminated public water pump.