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
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
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
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.
