index case

The first documented case of an infectious disease or genetically transmitted condition or mutation in a population, region, or family. It may also refer to an individual who has a disease, condition, or mutation that is the first one identified in a population. This second sense is synonymous with index patient. A related term is patient zero, “a person identified as the first to become infected with an illness or disease in an outbreak.” Patient zero is especially used to refer to a person documented as being the first known case of a communicable disease in a particular population or region.

» COVID-19 Glossary