When you bareback with random partners, there are two factors that affect whether you get HIV.
The first factor is the transmission rate from serodischordant bareback sex (sex where one partner has it and the other doesn't).
- The risk of contracting HIV by being a bareback bottom ranges from 8/1000 to 3/100. This latter figure means that it takes an average of only 33 fucks with an HIV top for you to get the virus.
- For tops, the risk is slightly lower, about 1/1000. Still, if you fucked 100 times with an HIV bottom, you would on average have a 1/10 chance of having gotten HIV.
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The second factor is the prevalence of HIV among the people you fuck. Now, it is fairly intuitive that since people who regularly bareback are at higher risk for HIV and other STDs, they likely have a higher than average HIV infection rate. Even if you and all your partners check regularly, this affords only a small measure of protection--a negative test only means that you didn't have HIV 3 months ago, since there is an average of a 3 month delay between getting infected and having a positive test.
For some hard data, I turned to the CDC. In a study conducted in 2004-2005 in 5 cities, they randomly screened gay men for HIV (the sites of recruitment were bars, nightclubs, gay community organizations, and street locations in gay districts)
Baltimore 40% infected, 62% didn't know
LA 19% infected, 42% didn't know
Miami 18% infected, 46% didn't know
NYC 18% infected, 52% didn't know
SF 24% infected, 23% didn't know
18-24 14% infected, 79% didn't know
25-29 17% infected, 70% didn't know
30-39 29% infected, 49% didn't know
40-49 37% infected, 30% didn't know
? 50 31% infected, 34% didn't know
White 21% infected, 18% didn't know
Black 46% infected, 67% didn't know
Hispanic 17% infected, 48% didn't know
Multiracial 19% infected, 50% didn't know
Other 13% infected, 50% didn't know
Average: 25% infected, 48% didn't know
Of note, 92% of the men had previously been tested, 64% in the preceding year.
Lessons: a negative test doesn't mean you know you don't have it if you continue to practice risky sex
Putting the whole picture together, 1 in 4 gay men have HIV, and 50% don't know. This means that if you only have sex with people who think they are negative, or don't know, and assuming no one lies, you have a 1 in 7 chance of having sex with someone with HIV when you meet casual partners.
What these statistics don't emphasize is that HIV infections are concentrated in the most promiscuous gay men, so the infection rate amongst people who bareback regularly are probably much higher. However, by recruiting study participants in cruisier areas, they probably overestimated the true prevalence of HIV among gay men. Ultimately, the true risk BB bottom who gets fucks by randomly selected, self-reported HIV- casual partners probably lies somewhere between the 3/100 I showed above, and the 3/700 you would expect from the prevalence data above. Suffice it to say, even 3/700 (1/233) is a very high risk (you would seroconvert within 4 years if you hooked up once a week).
Please keep in mind, these rates are averages. IT IS COMPLETELY POSSIBLE TO GET HIV FROM A SINGLE ACT OF UNSAFE SEX WITH AN HIV INDIVIDUAL!