Index case

The index case or patient zero is the first documented patient in a disease epidemic within a population,^[1] or the first documented patient included in an epidemiological study.^[2] 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.^{[citation needed]} 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.^[3]