https://www.poz.com/legacy/poz_magazine/articles/2014/p195_risk_chart.jpg
Probabilities of HIV transmission per exposure to the virus are usually expressed in percentages or as odds (see chart at the end of this article). For example, the average risk of contracting HIV through sharing a needle one time with an HIV-positive drug user is 0.67 percent, which can also be stated as 1 in 149 or, using the ratios the CDC prefers, 67 out of 10,000 exposures. The risk from giving a blowjob to an HIV-positive man not on treatment is at most 1 in 2,500 (or 0.04 percent per act). The risk of contracting HIV during vaginal penetration, for a woman in the United States, is 1 per 1,250 exposures (or 0.08 percent); for the man in that scenario, it’s 1 per 2,500 exposures (0.04 percent, which is the same as performing fellatio).
As for anal sex, the most risky sex act in terms of HIV transmission, if an HIV-negative top—the insertive partner—and an HIV-positive bottom have unprotected sex, the chances of the top contracting the virus from a single encounter are 1 in 909 (or 0.11 percent) if he’s circumcised and 1 in 161 (or 0.62 percent) if he’s uncircumcised. And if an HIV-negative person bottoms for an HIV-positive top who doesn’t use any protection but does ejaculate inside, the chances of HIV transmission are, on average, less than 2 percent. Specifically, it is 1.43 percent, or 1 out of 70. If the guy pulls out before ejaculation, then the odds are 1 out of 154.
NB:
[SIZE=3]Transmission can occur after one exposure.
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It is important to emphasize that a person could become infected from having unprotected sex once or a person could have unprotected sex many times and not become infected, regardless of how low or high the risk per exposure is.
A risk of 1% would mean that an average of one infection would occur if 100 HIV-negative people were exposed to HIV through a certain type of sex. It does not mean that a person needs to be exposed 100 times for HIV infection to occur.
REMEMBER:
During sex, our risk perception is replaced by love, lust, trust and intimacy.
www.catie.ca/en/pif/summer-2012/putting-number-it-risk-exposure-hiv