Outbreak investigation

Outbreak investigation is the systematic study of an unusual increase in illness to identify its cause, scale, transmission pattern, and means of control.

Case definition: A set of criteria used to classify people as cases in a particular investigation. A consistent case definition determines which illnesses count toward the outbreak.
Epidemiology: The study of the distribution and determinants of health-related states in populations. Outbreak investigations apply epidemiologic methods to a concentrated rise in illness.
Legionnaires' disease outbreak: An outbreak of pneumonia caused by Legionella bacteria, often linked to contaminated water systems. The 1976 Philadelphia outbreak illustrates how an unknown source can be traced through environmental evidence.
Endemic disease: A disease that is consistently present at an expected level in a population or area. A stable endemic baseline differs from the unexpected excess that defines an outbreak.
Epidemic curve: A graph showing the number of new cases over time during an outbreak. The curve can suggest when exposure occurred and how transmission unfolded.
Outbreak: An occurrence of disease cases exceeding what is normally expected in a defined place or group. An investigation begins by determining whether observed cases exceed the expected level.
2011 Germany E. coli O104:H4 outbreak: A large 2011 outbreak of Shiga toxin-producing Escherichia coli associated with fenugreek sprouts. Its investigation combined patient interviews, laboratory findings, and food-chain tracing.
Pandemic: An epidemic that spreads across multiple countries or continents and affects many people. An outbreak investigation can address a local cluster, while a pandemic spans many jurisdictions.
Descriptive epidemiology: The analysis of disease patterns by person, place, and time. These patterns help investigators locate affected groups and generate hypotheses.
Incidence: The occurrence of new cases in a population over a specified period. Incidence helps distinguish a genuine increase from a high but stable case count.

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