Case definition

A case definition is the standardized set of criteria that public-health authorities use to decide whether a specific individual counts as a case of a particular disease. Case definitions typically include clinical criteria (the symptoms a person must have), laboratory criteria (the diagnostic tests that must be positive), and epidemiological criteria (links to known cases, locations, or exposure events). They are the operational foundation of all case counts.

Most diseases have multiple case definitions running in parallel. WHO publishes 'suspected', 'probable', and 'confirmed' definitions for outbreak-prone diseases — Ebola, Marburg, cholera, hantaviruses, and others. National health ministries may adopt these directly or modify them to local surveillance capabilities. The same disease may be counted differently across countries because of case-definition differences, which is one reason cross-border case counts are not always directly comparable.

Case definitions are revised mid-outbreak as the disease's clinical profile becomes clearer and as testing capacity scales. When a definition broadens or narrows, the historical case count typically shifts — old cases may be reclassified, or new ones may suddenly become eligible. This is normal practice and one reason why reading outbreak case counts requires also knowing which case definition produced them.