Biologic factors in the sexual transmission of human immunodeficiency virus

The probability that any single episode of genital-genital or anogenital sexual intercourse will result in transmission of HIV may be determined by multiple biologic factors of the infectious person, the virus itself, and the exposed susceptible person. Some of these factors are known or suspected (figure 1), and they may explain observed differences in the sexual transmission of HIV in different parts of the world, notably in Africa, where genital ulcerative disease is probably influencing the epidemiology of HIV. Several studies have shown that infection in partners of HIV-infected persons is not determined solely by numbers of sexual encounters; on the contrary, HIV-infected partners have usually had fewer sexual encounters with infectious mates than have noninfected partners. Thus, sexually active persons should be cautioned that, to our knowledge, there are no nonsusceptible persons and that any single unprotected sexual encounter may lead to HIV transmission. Research into biologic factors that modulate HIV transmission continues to be hampered by difficulties in identifying HIV transmitters and nontransmitters, infective and noninfective variants of HIV (if the latter exist in vivo), and persons relatively more or less susceptible to HIV infection. However, as the number of partner studies and the number of those enrolled in them increase, a progressively clearer idea of the biologic determinants of sexual transmission of HIV should emerge.