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
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