The index case or primary case is the initial patient in the population of an epidemiological investigation. The index case may indicate the source of the disease, the possible spread, and which reservoir holds the disease in between outbreaks. The index case is the first patient that indicates the existence of an outbreak. Earlier cases may be found and are labeled primary, secondary, tertiary, etc. "Patient Zero" was used to refer to the index case in the spread of HIV in North America.
Read more about Index Case: Gaëtan Dugas Case ("Patient Zero"), Other Index Patients, In The Media
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