A disease outbreak is a recurring pattern of cases (suspected or confirmed) of the same illness that appears to be more than expected for the location and season. It may be caused by a single pathogen or several and can affect a small group of individuals or multiple groups within a region, nation or continent.
An outbreak is identified when increased numbers of cases are identified in surveillance systems, compared to the normal rates for that location and disease and excluding milder illnesses such as those resulting from food poisoning or other causes of illness that do not need to be diagnosed by laboratory methods. However, there are many possible influences on an assessment of an outbreak that include changes in reporting practices or improvements in diagnostic procedures and screening campaigns (detection bias) or interest in a particular disease by the media or public health authorities (sensationalisation bias).
The investigation of disease outbreaks involves methods of descriptive and analytical epidemiology. Often a case-control study is the study design applied. This involves the identification of a cohort of cases and comparison of their characteristics with those of healthy people in an unaffected population (controls). A nested case-control study is also used under certain circumstances.
Investigation of an outbreak of a disease can result in the establishment of general control and prevention measures that can be implemented. For example, food that is contaminated with the disease agent can be taken off the market, sick individuals who commercially have to handle the manufacturing or processing of foods can be prevented from doing so, or information can be distributed about risk-bearing activities.
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