index case

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

Keep scrolling for more

Learn More about index case

Time Traveler for index case

Time Traveler

The first known use of index case was in 1936

See more words from the same year

Statistics for index case

Last Updated

16 Mar 2020

Cite this Entry

“Index case.” Dictionary, Merriam-Webster, Accessed 29 Mar. 2020.

Keep scrolling for more

More Definitions for index case

index case


Medical Definition of index case

1 : an instance of a disease or a genetically determined condition that is discovered first and leads to the discovery of others in a family or population
2 : index patient we first examined the index case when he was aged 59 years— Stephanie B. Matthews & Anthony Campbell

Comments on index case

What made you want to look up index case? Please tell us where you read or heard it (including the quote, if possible).


See Definitions and Examples »

Get Word of the Day daily email!

Test Your Vocabulary

March 2020 Words of the Day Quiz

  • field of crocuses
  • Which is a synonym of rectitudinous?
Spell It

Can you spell these 10 commonly misspelled words?

Syn City

Test Your Knowledge - and learn some interesting things along the way.

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!