index case

noun
plural index cases

Definition of index case

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 case— K. 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

First Known Use of index case

1936, in the meaning defined at sense 1

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Time Traveler for index case

Time Traveler

The first known use of index case was in 1936

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index card

index case

index center

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Cite this Entry

“Index case.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/index%20case. Accessed 27 Nov. 2021.

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More Definitions for index case

index case

noun

Medical Definition of index case

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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