Patient Zero

Gabriel Gray, Chandra Suresh's supposed Patient Zero

In medicine, Patient Zero (also called the "index case") refers to the original source of an outbreak of a contagious illness within a population (that is, the origin of the outbreak from whom all other infections are ultimately derived) or to the most significant case within the population.

Chandra Suresh told his son Mohinder that he believed he was close to discovering the "Patient Zero" for his theories about new human evolutionary traits. Suresh's research indicates that his identified "Patient Zero" was Sylar.

In fact, Chandra had already assembled a list of identified evolved humans, and Sylar, then known as Gabriel Gray, was the first he chose to contact. His reasons for choosing Gray as his first subject are unknown.

Notes

• Chandra's actions in Six Months Ago and the fact that he had already identified many other potential evolved humans indicate that it's quite likely that Sylar was merely the first evolved human he contacted in New York. If by "Patient Zero" Chandra merely meant that Sylar was his first confirmed case, then he is using the term in quite a different sense than it was intended.
• It is possible that his use of the term "Patient Zero" might have some greater significance. Typically, referring to someone as "Patient Zero" would imply that they are the earliest case or the case from which other cases developed. Given that there are people on the list who are quite likely older than Sylar, it's unlikley that Suresh meant that Sylar was the first individual to develop powers or was the earliest-born individual with powers. However, it's still possible, though unlikely, that Chandra believed Sylar was pathologically responsible for others developing their powers.