index case

[in-deks keys]

noun

Pathology.

1. the first documented instance of a disease or other medical condition in a population, especially the first infection in an epidemic.

2. (loosely) index patient.

index case

noun

1. med the first case of a disease, or the primary case referred to in a report

Word History and Origins

Origin of index case¹

First recorded in 1935–40