[smashkins]

I find this quite interesting. Just to share:

What about the "per-act risk" numbers?

Epidemiologists – scientists who study the transmission and control of epidemic diseases – are fond of using mathematical models to calculate the risks of certain infections spreading (like HIV) from person to person. These calculations are usually written as the "per-act" or "per-contact" risk. For example, one study conducted by the CDC – and frequently cited by many sources of HIV transmission information – suggested that a person has a 1 in 200 chance of becoming infected with HIV if he is the receptive partner during unprotected anal intercourse with someone known to be HIV positive. On the other end of the spectrum is the risk calculated for insertive oral sex: 1 in 20,000 if condoms are not used and the receptive partner is known to be HIV positive.

Making personal decisions about unprotected sexual activity using these numbers is not recommended. For starters, these numbers are based on assumptions and guesses, not actual evidence of HIV transmission. Second, even if the researchers are right – that there is a 1 in 20,000 chance of being infected with HIV via unprotected insertive oral sex with someone known to be positive – many experts argue that this number is close enough to zero as to not be concerned about it at all.

Just as these numbers can unnecessarily scare people, they can also give some people a false sense of security. For example, the 1 in 200 risk associated with receptive anal sex with someone known to be HIV positive doesn't mean that you can have unprotected anal sex 199 times before becoming infected. It's very possible to become infected with HIV after just one incident of unprotected receptive anal intercourse.

It's also important to stress that there are a number of factors than can influence the risk of HIV transmission. Because there are so many factors, it's virtually impossible to interpret per-act or per-contact risk numbers when making personal decisions about sexual activity (or looking back on previous decisions). For example, HIV-positive people with very high viral loads (high amounts of HIV in their blood and genital secretions) may be more likely to transmit the virus to their partners, during unprotected insertive anal, vaginal, or oral sex. The presence of STDs, especially ulcerative STDs, can increase the risk of spreading (or becoming infected with) HIV. Trauma – such as abrasions or cuts inside the vagina, anus, mouth, or on the penis – can also increase the risk of transmission. All of these can increase the per-act risk of HIV transmission.