index case
Americannoun
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the first documented instance of a disease or other medical condition in a population, especially the first infection in an epidemic.
-
(loosely) index patient.
noun
Etymology
Origin of index case
First recorded in 1935–40
Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
What’s different this time is that the couple who are considered the index case went bird-watching in Chile, Argentina and Uruguay before boarding the cruise ship, according to the World Health Organization.
From MarketWatch ● May 7, 2026
They include the likelihood that Ebola was spreading weeks before the index case was identified and that patients initially sought care at healthcare facilities with sub-optimal infection-control practices.
From Los Angeles Times ● Oct. 17, 2022
Our investigations have not yet been able to identify the index case.
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
G. was the first identified case, the index case, in an outbreak of an unknown virus.
From "The Hot Zone" by Richard Preston
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
