An outbreak is a sudden increase in the number of cases of a disease compared to the expected amount in a community, geographical area or season. A disease outbreak is maintained by infectious agents that spread from person to person, from animal to person or environment to person, and almost always through human behavior. The human behaviour that facilitates transmission is what distinguishes disease outbreaks from endemic and epidemic diseases that are usually maintained by natural processes such as the presence of a virus or bacteria in an environmental medium or the existence of a vector.
Successful outbreak investigations require the simultaneous use of epidemiological, microbiological and toxicological methods. The aim is to develop and test hypotheses that will inform decisions on appropriate interventions and preventive measures.
A critical first step is to establish a case definition which specifies the persons, time and place associated with the disease. This will help to limit the number of people who are evaluated for a diagnosis of an outbreak. A more refined case definition will allow for the exclusion of other illnesses with similar symptoms (e.g., norovirus or atypical E coli) and will enable the identification of individuals who have had laboratory-confirmed illness caused by a particular pathogen.
The next step is to evaluate the data in terms of whether an outbreak is occurring. The evaluation should take into account any biases which may impact the results. These could include, for example, changes in reporting practices, improved diagnostic procedures or screening campaigns and increased interest of the public and media in a particular disease.
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