A disease outbreak is a group of cases that occur in excess of what would be expected in a given community, geographic area or season. Outbreaks can be caused by infectious agents (bacteria, viruses and fungi) spread directly from person to person or indirectly from animal reservoirs to people through insect vectors or environmental sources. Human behavior often contributes to the rapid spread of these diseases, and early detection, response and prevention are essential for minimizing their negative social and economic impact.
A case definition identifies individuals with an illness that is linked to the outbreak under investigation. The case definition must be specific to the disease of interest and include criteria such as: Person: Identify the affected individuals (e.g., age, sex, gender). Place: The particular geographic location associated with the outbreak (e.g., county, state, city). Time: Specify the time period associated with illness onset for cases under investigation (e.g., 3 weeks after consuming contaminated water). Clinical features: The characteristic signs and symptoms of the disease (e.g., fever, cough, sore throat).
The preferred study design in an outbreak investigation is a case-control study. In this approach, the risk of developing an infection is compared among cases and a non-case control population. This is the most efficient way to determine whether an exposure is causing disease. However, it is not always feasible to recruit an independent sample of the population at risk for an outbreak. If this is the case, a nested case-control study should be conducted within a cohort with a defined risk.