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 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

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

Word History

First Known Use

1936, in the meaning defined at sense 1

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