Three factors were responsible for the original concentration of AIDS in the male homosexual population in the United States. First, it appears that AIDS originated in Africa as a mutation of an endemic virus and was carried from there to Haiti. Haiti was a popular vacation spot for homosexuals, and male prostitution was widespread among the impoverished Haitians. Irrespective of how the initial introduction of the disease occurred, the second factor was the existence of the bathhouses. These provided large numbers of sexual contacts and a high incidence of other sexually transmitted diseases. It is likely that genital lesions secondary to these other venereal diseases made the spread of HIV easier. The third factor was that the HIV subtype initially introduced into the United States is more difficult to spread heterosexuality.

Most important, the rate of spread of diseases is proportional to the frequency of contact with potentially infected individuals and the effectiveness of those contacts in spreading the disease. This becomes critical for diseases such as HIV with a low probability of transmission in a given sexual encounter. Some bathhouse patrons had more than 1000 sexual contacts a year. A very sexually active heterosexual man might have 100 contacts a year. Although HIV was probably introduced into the heterosexual population simultaneously as in the homosexual population, the spread would be less than one-tenth as fast. (Although female prostitutes do have large numbers of sexual contacts, women are less effective at infecting their sexual contacts than are men.)