Unprotected vaginal intercourse is the most common mode of HIV infection worldwide. In the United States and many other developed nations, it is the second most common mode of sexual HIV transmission (after anal intercourse among men who have sex with men).
Studies have consistently found that HIV-positive men are much more likely to transmit the virus to HIV-negative women through vaginal intercourse than HIV-positive women are to HIV-negative men. There are a few reasons for this. First, there are more men than women in the United States infected with HIV (i.e., it's more likely for a female to have sex with an HIV-positive male than for a male to have sex with an HIV-positive female). Second, women have a much larger surface area of mucosal tissue than men. Mucosal tissue lining the vagina and cervix can chafe easily and are rich in immune system cells that can be infected by HIV. For men, HIV must enter through a cut or abrasion on the penis, or through the lining of the urethra inside the penis.
There has been some research suggesting that men who are uncircumcised have a higher risk of becoming infected with HIV or transmitting the virus. However, it is important to stress that men who are circumcised can still be infected (or transmit the virus) if condoms are not used for vaginal sex.
Men or women who have ulcerative sexually transmitted infections (STIs), such as genital herpes or syphilis, are more likely to spread the virus if they are HIV positive, or to become infected with the virus if they are HIV negative.
To reduce the risk:
Correctly and consistently use latex or polyurethane condoms every time you have vaginal intercourse with a partner who is positive or whose HIV status you do not know.
Use a water-based or silicone-based lubricant with latex condoms. Lube keeps condoms gliding smoothly, reducing the risk of rips and tears. Lubrication also helps protect the vaginal wall from rips and tears, which can increase the risk of transmission.
Don't douche before engaging in vaginal intercourse. This can destroy the healthy bacteria in the mucosal lining of the vagina and can eliminate the vagina's natural lubrication.
Don't engage in unprotected vaginal intercourse during menstruation.
