index case

noun

plural index cases
1
: the first documented case of an infectious disease or genetically transmitted condition or mutation in a population, region, or family
There were 54 cases of meningococcal disease in the 22 cluster schools, 30 of which occurred 1 or more days after the index caseK. M. Zangwill et al.
2
: an individual whose infectious disease or genetically transmitted condition or mutation is the first identified in a population, region, or family : index patient
The index case was identified as a 20-year-old female student who had spent the winter break in a California county where there was a widespread community outbreak of measles. She returned to campus and became ill four weeks before the first case was reported.William A. Christmas et al.
A test of this model would require a more extensive family study, one that included the investigation of parents and other relatives, in addition to siblings, of the autistic index cases.Susan E. Folstein
compare patient zero

Examples of index case in a Sentence

Recent Examples on the Web Further, these pediatric index cases were reduced by 60% to 80% during school breaks. Libby Richards, The Conversation, 9 Aug. 2024 But in only a handful of cases has the transmission of these evolved variants from index cases that have been well characterized been observed. Elizabeth Cooney, STAT, 24 Jan. 2024

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'index case.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History

First Known Use

1936, in the meaning defined at sense 1

Time Traveler
The first known use of index case was in 1936

Dictionary Entries Near index case

Cite this Entry

“Index case.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/index%20case. Accessed 5 Nov. 2024.

Medical Definition

index case

noun
1
: the first documented case of an infectious disease or a genetically transmitted condition or mutation in a population, region, or family
2
: an individual whose infectious disease or genetically transmitted condition or mutation is the first identified in a population, region, or family : index patient
We first examined the index case when he was aged 59 years …Stephanie B. Matthews and Anthony Campbell
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