An outbreak is a sudden increase in the number of cases of an infectious disease in a community or geographic area. This increase is in excess of what health officials expect to see, and may be a result of an introduction of a new pathogen (e.g., cholera, rotavirus, or SARS-CoV-2 virus) or the resurgence of an existing pathogen such as hepatitis A or E. Generally, an outbreak is not considered a public health emergency, but it is often a serious concern for people’s lives and well-being. An outbreak is different from an epidemic, which is a large-scale occurrence of disease in an area of the world that exceeds what could be expected from the size and type of previous and existing exposure to the disease.
In general, an outbreak can be detected by comparing the current number of cases of a disease with the expected number of cases for that period. This information can be obtained from health department surveillance records, disease registries, or hospital discharge data. However, variations in reporting procedures or sudden changes in population size can influence the number of cases that would indicate an outbreak.
Public health institutions need to perform a wide variety of activities during an outbreak, and these activities must be prioritized in light of available resources, including staff, budgets, and public receptiveness. A review of the literature on outbreak response activities found that most settings require a core set of response actions, which are common to both epidemics and non-epidemics.