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The first American AIDS patient - Patient Zero
Since HIV and AIDS appear to have originated in Africa, someone or perhaps a group of people brought it to the United States where it entered the gay community. Presumably, the association of AIDS with homosexuality is quite fortuitous since in third world countries AIDS shows no sexual preference. It just happened that the person or persons who brought HIV to the United States was gay.
There has often been an allusion to a "patient zero" who introduced AIDS to the gay community of California. In fact, the patient was not called zero at all but was designated by the letter O (for Out of California) but the name patient zero persisted.
The idea came about in a study of a cluster of cases in California that appeared to be linked by sexual contact, and at the center of this cluster was a man called Gaetan Dugas, a Canadian flight attendant. However, as pointed out by Dr Andrew Moss *, the linking of infections in the cluster to one central patient was based on the erroneous assumption at that time that the incubation period from infection to the manifestation of disease was about a year, whereas we now know that a decade is more usual. In view of this the apparent cluster dissolves and there is no evidence that the late Mr Dugas was the person who brought HIV into the United States. There never was a "patient zero".
* New York Review of Books December 1988 (http://www.nybooks.com/articles/4227)