concerning anal sex.

It is my understanding that this is the most common form of transmission of HIV. My question is who is more at risk, the man or the woman? Or for that matter the same question if it were of the same sex.

Actually, anal sex is not necessarily the most common form of transmission. In the United States, the majority of HIV infections comprise men who have sex with men (MSM) and injection drug users. It is assumed that among MSM, transmission most often occurs through unprotected anal sex, and probably more often from the top to the bottom (or insertive to receptive). This is only in the United States. Worldwide, heterosexual sex (which includes both vaginal and anal) accounts for the vast majority of HIV infections. There are an estimated 34.3 million people living with HIV/AIDS, 95% living in sub-Saharan Africa. The epidemic is much different in Africa than in the U.S. For instance, African men seem to have a much higher chance of getting infected through insertive vaginal sex than men in the United States. See Who gets AIDS? for more information on statistics.

HIV may be more easily transmitted through anal sex than vaginal sex for a few reasons.

1) The anal/rectal membrane tends to be more susceptible to micro-trauma (small tears in the membrane) which would facilitate transmission.

2) Some studies suggest that certain types of cells more prone to infection reside in the mucosal lining of the rectum.

3) Due to trauma, there is a greater likelihood for blood to be present during anal sex, increasing the chance for transmission to the top/insertive partner.

In general, receptive partners during sex (ones who are more likely to get more infected fluid in their own body) are the ones at most risk. But remember, there are a lot of variables that effect transmission. Use a condom and water-based lubricant to reduce your chances.

RMK