HIV/AIDS epidemiological terms-of-art.
In HIV/AIDS epidemiology, prevalence indicates the percentage of men infected with HIV within a given study cohort.
For example, if 25 men are infected with HIV in a cohort of 100 men aged 15-49 who are being studied for antibodies to hepatitis B, HIV prevalence in that cohort is 25%.
Prevalence is contrasted with incidence, which is the percentage of men within the cohort who become HIV positive in the space of a given year.
So, if 15 of the same men being studied become HIV positive over a year's time, within that study cohort, incidence is 15%, and prevalence is 25%.
In the classic San Francisco hepatitis B study, and as described by UC Berkeley epidemiologist James Chin, "HIV prevalence rose from 1 percent in 1980 to 25 percent in 1982 to 65 percent in 1984."
While "annual HIV incidence peaked in 1983 at over 20 percent and then decreased sharply and dropped to about 5 percent by 1985."
Why?
Because by 1985, virtually all those who *behavior* put them at risk for HIV infection -- had been infected.
And the behavior, among sexually-active msm -- is anal.
At least 90% of all sexually-transmitted cases of HIV among msm are attributed to anal, with most researchers believing the percentage is closer to 99%.
The remaining cases are attributed to oral.
Not even one case of documented HIV transmission, by contrast, has ever been attributed to Frot.