index case
Americannoun
-
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
“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
At least twice, those infections could have spread from an index case to other people, he says.
From Scientific American ● Feb. 28, 2022
These contacts - people who the "index case" has been in close physical proximity to for long enough to potentially infect them - are then told to isolate and book a test of their own.
From BBC ● Jul. 1, 2021
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.
