Patient Zero An Epidemic An Outbreak The Prime Mover 
Answer: Patient Zero
In epidemiological investigations, the initial patient in a population is referred to as the “index case”. This person is not necessarily the absolute first person to have the illness or infection, but may only be the first person to bring an illness into a new population. If, for example, there was an extremely virulent strain of the flu brought into North America on a flight from Asia, it might be possible to locate the original airline passenger who carried the virus over.
Since the 1980s, index cases, especially in regard to highly contagious outbreaks, have been widely referred to as “patient zero” in both the news and in creative works like novels, films, and television. The concept of “patient zero”, for example, has featured prominently in films like Contagion, Outbreak, and World War Z, and gets frequent use in a wide variety of television dramas.
While the term makes perfect sense, the term actually came into use as a result of a misprint. In a study conducted back in the 1980s on the then-recently-identified AIDS epidemic, a French Canadian airline steward named Gaëtan Dugas was given the identifier “Patient O” in the data set—that’s O as in Orange. The O designation was simply intended to note in the California-based study that Dugas was a patient from outside the geographic region of the study.
The study and subsequent analysis of it suggested (we now know erroneously) that Dugas was the index case for the AIDS outbreak in the United States. When published, the “Patient O” identifier was mislabeled as “Patient 0” (the letter transposed with the very similar numeral). The term “patient zero” was used in reference to him in both scientific literature on the subject and in the popular press, and the term caught on with the public to indicate the first patient with an infectious disease.