index case

American  

[in-deks keys] / ˈɪn dɛks ˌkeɪs /

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.

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index case British  

noun

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

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Etymology

Origin of index case

First recorded in 1935–40

Example Sentences

We can’t determine the index case until we confirm it.

From Scientific American • Oct. 13, 2022

“There is some evidence that passengers within two rows of an index case are at higher risk,” said Patricia Schlagenhauf, a professor of travel medicine at the University of Zurich.

From Seattle Times • Mar. 22, 2022

However guidance published by the WHO online appears to suggest that its goal is for 80% of the close contacts of each index case to be in quarantine within three days of them developing symptoms.

From BBC • Jul. 1, 2021

In the cases where they could identify an index case in the small number of what seemed like in-school transmissions, 80% of it involved a staff member.

From Slate • Mar. 18, 2021

G. was the first identified case, the index case, in an outbreak of an unknown virus.

From "The Hot Zone" by Richard Preston