Living with HIV/AIDS  

Sexual Transmission of HIV

Unprotected penile-anal intercourse (male-male or male-female)

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.

Unprotected penile- vaginal intercourse

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’ 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.
While both men and women are at risk of contracting HIV during heterosexual intercourse, the woman is at greater risk, for the following reasons (6):

  • 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).
  • The vagina and cervix have larger areas of exposed mucous membranes than does the penis.
  • The tissues of the vagina and cervix are easier to tear than the penis.
  • The virus has an easier time surviving in the vagina than it does on the surface of the penis (7, 8).
  • There are more copies of the virus in a man’s semen than there are in the fluids of the vagina (8).

Unprotected oral-genital sex

Unprotected oral-genital sex exposes HIV-infected semen or vaginal fluids to oral mucous membranes. While this is a biologically possible means of HIV transmission, the actual risk of it is unknown. The risk of oral-penile contact is thought to be low, and the risk of oral-vaginal contact is thought to be very low (1).

Factors That Increase the Risk of Sexual Transmission

  • Sexual behavior that is accompanied by bleeding, such as whipping, cutting, or piercing skin during sex. While these sexual practices may transmit infection, so few data are available that it is difficult to know how much they increase the risk of HIV transmission (5).
  • Multiple sexual partners. The more HIV+ people in a pool of sexual partners, the greater the chances that a person will encounter an HIV+ person and contract HIV himself or herself (9). For this reason, the risk associated with having multiple sex partners varies significantly by geographic region, and by the sexual mixing within a region (5,10).
  • Other sexually transmitted infections. Having another STI/STD greatly increases the risk of getting or giving HIV (5). This is especially true of the STI/STDs that cause sores on the genitalia. Some STI/STDs that do this are gonorrhea, chlamydia, herpes, and syphilis. Even if an STI/STD does not cause genital sores, though, it still increases the risk of HIV transmission and acquisition, by increasing the number of CD4+ cells near the genitalia.

1. Kalichman, S.C. Preventing AIDS. A Sourcebook for Behavioral Intervention. Mahwah, NJ: Lawrence Erlbaum Associates, 1998.

2. Chin. J. (Ed.) Communicable Diseases Manual, 17th Edition. Washington, DC: American Public Health Association, 2000.

3. Institutes of Allergy and Infectious Diseases (NIAID). HIV Infection and AIDS: An Overview. National Institutes of Allergy and Infectious Diseases, National Institutes of Health, U.S. Department of Health and Human Services, October 2003. Retrieved on January 14, 2004 from

4. Winkelstein W. Jr., Lyman, D.M., Padian, N., Grant, R., Samuel, M., Wiley, J.A., Anderson, R.E., Lang, W., Riggs, J., & Levy, J.A. Sexual practices and risk of infection by the human immunodeficiency virus: The San Francisco Mens Health Study. JAMA 257:321-325,1987.

5. Osmond, D.H. Sexual Transmission of HIV. HIV InSite Knowledge Base Chapter. University of California, San Francisco, 1998. Retrieved on January 14, 2004 from

6. United Nations Population Fund (UNFPA). Addressing gender perspectives in HIV prevention, HIV prevention now, programme briefs, No. 4. United Nations Population Fund, 2002.

7. Editors. The most common opportunistic infections in women with HIV. HIV Newsline, 4 (4), 1998.

8. World Health Organization (WHO). Women and HIV/AIDS (Fact Sheet No. 242). Geneva: World Health Organization, 2000. Available online at:

9. Laumann, E.O., Gagnon, J.H., Michael, R.T., Michaels, S. The Social Organization of Sexuality: Sexual Practices in the United States. Chicago, Illinois: The University of Chicago Press, 1994.

10. Zierler, S. & Krieger, N. Social Inequality and HIV Infection in Women. In The Emergence of AIDS: The Impact on Immunology, Microbiology, and Public Health, K.H. Mayer & H.F. Pizer (eds.). Washington, DC: American Public Health Association, 2000.

© Sociometrics Corporation, 2004