An outbreak is an increase in cases of a disease that is greater than the expected number for a specific location or population over a specified time period and that may be associated with a common exposure. Outbreaks are often sudden.
An investigation of an outbreak is the process of identifying and explaining how the disease occurred and how it spread. It is a crucial component of public health practice. The work of epidemiologists like Edward Jenner, who developed a vaccine for smallpox, and John Snow, who discovered that contaminated water was the cause of cholera in London in 1850, laid the foundations of modern outbreak investigations.
The first step in an outbreak investigation is to identify and understand the number of disease cases that would be expected for a specific location or population over e a specified time period. This information is usually available through various sources such as health department surveillance records, disease registries and hospital discharge records.
Once the expected number of cases is identified, an investigation can begin. The investigation may involve a combination of approaches. The most important of these is to find and evaluate the potential causes for the increased number of disease cases.
During an outbreak, people who are sickened with similar symptoms may be interviewed by local health departments (LHDs) to collect and assess health history, symptoms and possible exposure information. The LHD that serves the city or county where the ill persons live or work may interview ill and non-ill individuals in person, by telephone or through a website (under 12VAC5-90-90.d of the Virginia Regulations for Disease Reporting and Control).