The
index case,
primary case, or
patient zero is the initial
patient in the
population of an
epidemiological investigation, or more generally, 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 will sometimes achieve the status of a "classic" case in the literature, as did
Phineas Gage.