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.The index case may indicate the source of the disease, the possible spread, and which reservoir holds the disease in between outbreaks.