Index case

The index case or patient zero is the first documented patient in a disease epidemic within a population, or the first documented patient included in an epidemiological study. It can also refer to the first case of a condition or syndrome (not necessarily contagious) to be described in the medical literature, whether or not the patient is thought to be the first person affected. An index case can achieve the status of a "classic" case study in the literature, as did Phineas Gage, the first known person to exhibit a definitive personality change as a result of a brain injury.