HIV is transmitted primarily through sexual contact, parenteral exposure to blood or blood products, and perinatally from infected mothers to their infants.

Sexual Transmission

The predominant mode of HIV transmission throughout the world is sexual contact. The risk of acquiring HIV infection during a single sexual contact depends on several factors. Most important, of course, is the likelihood that the contact is with an HIV-infected partner. Because the prevalence of HIV varies widely between populations within countries as well as between countries, the rates of sexual transmission also vary. Other factors affecting the efficiency of sexual transmission include the type of sexual practice; the infectivity of the source partner; coexisting sexually transmitted infections in either partner, particularly those causing genital ulceration; and consistency of condom use. HIV transmission has been attributed to vaginal, anal and, less frequently, oral intercourse.

In epidemiologic studies among homosexual men, the risk of HIV acquisition increases with the number of sexual partners, the frequency of receptive anal intercourse, and practices associated with rectal trauma such as receptive “fisting” and anal douching. No sexual activity potentially involving the exchange of semen or blood, however, should be considered without risk. The relative efficiency of HIV transmission through various sexual practices was difficult to estimate precisely because most HIV-infected homosexual men in epidemiologic studies had engaged in multiple practices. Although the frequency of female-to-female transmission would seem to be quite low, such HIV infections associated with traumatic sexual practices have been reported. Most cases of HIV infection reported among bisexual women and lesbians are attributed to injecting drug use or heterosexual contact.

Most heterosexual transmission of HIV occurs during vaginal intercourse, although some studies suggest that receptive anal intercourse increases the risk of HIV transmission from an infected man to a woman. Some infected persons may be more efficient transmitters than others, perhaps owing to differences in viral strains or other factors. Transmission efficiency is inversely related to the immunologic status of the infected partner. In studies conducted among spouses and other steady sexual partners of HIV-infected persons, male-to-female, female-to-male, and male-to-male sexual transmission of HIV increased as the index partner’s T-helper lymphocyte numbers declined.

These findings are not surprising, because the quantity of HIV in blood and semen increases as the disease progresses and the immune system weakens. Several studies have documented that infections such as Haemophilus ducreyi, Treponema pallidum, herpes simplex virus, and other pathogens causing genital or anal ulcers facilitate acquisition or transmission of HIV through sexual contact, most likely by disrupting the genital or anal skin and mucous membranes. Undoubtedly, the higher rates of untreated genital ulcer disease contribute to the high rates of sexual transmission of HIV observed in some areas of the developing world. Several investigators have reported increased risks of HIV acquisition for women with cervical infections with Neisseria gonorrhoeae or Chlamydia trachomatis or with cervical ectopy. To the extent that coexisting sexually transmitted infections increase the rate of HIV transmission, populations throughout the world with higher rates of these infections are at higher risk of HIV infection.

Conversely, preventing and treating other sexually transmitted infections should have a beneficial effect on preventing HIV transmission. Cohort studies of couples discordant for HIV infection clearly indicate that consistent condom use reduces heterosexual as well as homosexual HIV transmission by
90% compared with inconsistent use or nonuse of condoms. Finally, preliminary studies suggest that antiretrovirals may reduce but not eliminate the risk of HIV transmission through sexual contact. However, it is unknown if treatment of HIV infection will lower infectiousness at a population level, there is a need to continue interventions that decrease sexual risk behavior and to treat sexually transmitted infections as known effective strategies that can lower the sexual transmission of HIV.