Abstract: According to a new study from the University of California, San Francisco, women are far more likely than men to be infected by an HIV-positive heterosexual partner. Nancy Padian, the study's lead investigator, noted in the current issue of the American Journal of Epidemiology that women were approximately eight times more likely than men to become infected by an HIV-positive partner. The researchers estimated that male-to-female unprotected heterosexual intercourse results in about nine infections to every 10,000 unprotected sexual encounters, while female-to-male transmission rates are even lower.
From a different study:
"For regular partners who were active in anal sex, the risk for transmission was 1 in 10. The risk for the insertive partner (the "top") is believed to be about 10 times less than for the receptive partner (the "bottom"). Another study showed 1 in 50 for receptive anal intercourse. [I assume here it is not considering 'regular activity' but a general risk]
The risk of HIV infection during vaginal intercourse is believed to be much less. One estimate was 1 in 200,000 for transmission from infected women to men and 1 in 100,000 for transmission from infected men to women"