Sexual transmission of HIV

Unprotected penile-anal intercourse, whether between two men or a man and a woman, exposes the anal and rectal mucous membranes of the receptive partner to semen, and exposes the penis (specifically, the mucous membrane in the opening of the penis) to anal mucus. Also, anal sex often causes tearing of the penis, anus, and rectum, so that both the receptive and the insertive partners may be exposed to blood. The anal and rectal tears, in turn, provide HIV with direct access to the bloodstream.

The receptive partner in anal sex (ie, the person who is being penetrated) is at greater risk of HIV infection than the insertive partner for several reasons. First, the receptive partner is exposed to a larger amount of body fluids (semen, possibly blood) than the insertive partner, who is exposed to only rectal mucus (possibly blood). In addition, the receptive partner is exposed to HIV-carrying fluids for longer than is the insertive partner, as the semen (and, perhaps blood) deposited into his or her rectum stays there longer than the fluids on the insertive partner's penis. HIV also has an easier time surviving in the rectum than on the penis. Finally, rectal and anal tissues tear more easily than do penile tissues, giving HIV a direct pathway to the bloodstream of the receptive partner (Osmond, 1998).

Unprotected penile-vaginal intercourse exposes the woman's vaginal and cervical mucous membranes to semen, and the man's penis (specifically, the mucous membrane at the penis’s opening) to vaginal secretions. Moreover, if the sex is rough or dry, cuts and tears may expose the man and woman to blood, and may open delicate vaginal and penile mucous membranes for direct HIV transmission to the bloodstream.

Although both men and women are at risk of contracting HIV during heterosexual intercourse, the woman is at greater risk for several reasons. Usually, the woman is exposed to a larger amount of body fluids (semen and possibly blood) than is the man, who is only exposed to vaginal secretions (possibly blood as well). In addition, the vagina and cervix have larger areas of exposed mucous membranes than does the penis. The tissues of the vagina and cervix also tear more easily than those of the penis, and the virus has an easier time surviving in the vagina than it does on the surface of the penis. Finally, there are more copies of the virus in a man's semen than there are in the fluids of the vagina (United Nations Population Fund [UNPFA], 2002; Volberding, 1998; WHO, 2000).

Unprotected oral-genital and oral-anal sex can expose mucous membranes of the mouth to HIV-infected semen, vaginal fluids, or anal fluids. Although this is a biologically possible means of HIV transmission, very few cases of transmission through these routes have been documented, and the actual risk of infection is unknown. The risks of oral-penile and oral-anal transmission are thought to be low, and the risk of oral-vaginal contact is thought to be very low (Campo et al, 2006; Kalichman, 1998).