A disease outbreak is a group of cases (typically two or more) of the same disease that are significantly more than expected and/or occur in close proximity in time and space. The presence of a disease in a person is not required for an outbreak to occur; a group of people may be infected with the same infectious agent and not experience the same clinical presentation of disease (WAC 246-101).
Typically, an investigation of a disease outbreak involves applying methods of descriptive and under some circumstances analytical epidemiology to identify the source, pathogen, and mode of transmission. Ultimately, these factors inform control and prevention actions.
Since the 18th century, when Edward Jenner first introduced smallpox vaccine and John Snow observed that contaminated water was the cause of cholera in London, the practice of investigating disease outbreaks has been a key component of public health. Today, technological advances such as regional syndromic surveillance systems and rapid diagnostic tests have enhanced global outbreak detection and response.
An important aspect of outbreak investigations is communication. Investigation results are typically reported to all relevant local, state, and federal public health departments, and the general community through news media. Moreover, the investigation process is not complete until preventive measures are implemented. These may include recalling contaminated products, closing facilities for manufacturing or processing, improving hygiene, blocking the spread of a disease via animate or inanimate environmental means, and education campaigns. These actions can be both immediate and long-term.