A disease outbreak is an increase in the number of cases of a certain illness (such as diarrhea, pneumonia or encephalitis) in a given area. It may affect a small and localized group or affect thousands of people in different countries around the world, depending on the pathogen, existing level of exposure to it and its mode of spread.
Epidemiology is the discipline that studies disease outbreaks, and it focuses on finding the source of the infections and how they spread. Traditionally outbreak investigations have been conducted in order to find the infectious agent responsible for an epidemic and to develop effective control and prevention measures.
An important part of an outbreak investigation is establishing case definitions. These should include criteria for place (e.g. state, county or city), facility associated with the outbreak (e.g. X nursing home, Y school), time of illness onset and clinical features, among others. Using these criteria, investigators can compare the cases under study to the expected incidence rate and determine whether an outbreak is occurring.
A large outbreak can be caused by a single point of exposure that exposes the entire population at risk to the same infection simultaneously, such as a foodborne or watery diarrheal disease. However, more often a series of smaller control outbreaks occur and the cause is found from one of these incidents. CDC investigators work with partners to investigate domestic and international outbreaks and disease clusters that are related to environmental exposures.