If one partner is known to be infected with the virus, what is the likelihood of transmitting the virus to the other partner during sexual intercourse?
This depends on the sex of the partners, the mode of sexual activity, the availability of safety precautions and the duration of infection of the infected person.
Generally speaking, the risk of sexual transmission is highest among gay men, followed by male to female, female to male, and female to female. Among the different sexual practices, anal sex has the greatest risk of transmission, followed by unprotected (i.e., condomless) vaginal sex, oral sex, and mutual masturbation. Specifically, the probability of transmission from unprotected (condomless) anal sex is between 1/1600 and 1/10, i.e., the most alarming statistic is that one in ten times of unprotected anal sex will result in the possibility of HIV infection, while the most optimistic statistic is one in 1600 times. The other risk probabilities are in this order, 1/2000 to 1/200 for men to women during vaginal intercourse, 1/3000 to 1/700 for women to men, and there are no reliable statistics for transmission between lesbians to date.
Researchers have also found that whether one “performs” or “receives” sexual intercourse is an important factor in oral, vaginal, and anal sex. The probability of infection is half as high for the recipient of oral sex as for the one who performs it; it is 10 times higher for vaginal penetration, 13 times higher for anal penetration, 20 times higher for vaginal penetration and 100 times higher for anal penetration than for the recipient of oral sex. For example, for men, the risk of contracting HIV from unsafe anal sex is 1 in 1,600 to 1 in 10, while using a condom reduces this risk to 2.5 in 100,000. In terms of probability, sexual transmission of HIV is not as easy as one might think. In particular, it is much less likely to be transmitted compared to traditional STDs.
However, this does not mean that sexual transmission of HIV can be ignored, as they say, “even a small chance of it happening to a particular person becomes 100 percent”. Therefore, cleanliness and safe sex are still the principles we should follow.