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

Example Sentences

Yet the suspected index cases — the people who likely spread the virus to others — tested negative for the virus a couple of days before boarding their plane in Switzerland and thought they weren’t infected.

From Vox

Investigators have also found that in cases of mass hysteria, one child is often an “index case” that triggers waves of anxiety.

From The Daily Beast