factors other than heterosexual intercourse, and seroconversion of male sex partners of infected women that occurred while the couples were being studied prospectively. With respect to the biological plausibility of female-to-male transmission, it has been argued that since other sexually transmitted diseases are bidirectional in nature, it is not unreasonable to assume that HIV can spread in the same manner. A number of studies have documented that African and Indian men who have multiple female sex partners or sexual contact with commercial sex workers are at highrisk for becoming infected with HIV (Cameron, Plummer, and Simonsen, 1987; Carswell, Lloyd, and Howells, 1989; Clumeck et al., 1985; Kreiss, Koech, and Plummer, 1986). The most persuasive case reports of female-to-male transmission have been those in which (1) the female acquired the infection from a transfusion or organ transplant and her male partner (without other known risk factors) subsequently seroconverted (L'Age-Stehr, Schwarz, and Offermann, 1985) and (2) a sequential chain of male-to-female-to-male transmission was observed (Calabrese and Gopalakrishna, 1986).

Although significant numbers of female-to-male infections have been documented in Africa (Barnett and Blaikie, 1992; Panos Institute, 1988; Shannon, Pyle, and Bashshur, 1991), such a mode of transmission has been reported only infrequently in the United States, and the majority of the more recent case reports have come from investigators in Europe (Beck et al., 1989; European Study Group, 1992; Johnson, Petherick, Davidson, et al., 1989; Lefrere et al., 1988). Several explanations have been offered for the differences in female-to-male transmission rates between the United States and certain countries in Africa. A number of researchers have suggested that the documentation of infrequent heterosexual transmission from women to men in this country may be a function of the history of the epidemic, since the initial phase was largely confined to men who have sex with men and injection drug users, so that during that time the number of infected women was low, and the possibility of female-to-male transmission was small. Because the majority of AIDS cases occurring today reflect infections that were acquired during the early years of the epidemic, most heterosexually acquired infections among men may still be in the asymptomatic or latent stage (Friedland and Klein, 1987; Osmond, 1990). Also, the fact that the infectivity of an HIV carrier increases over time may magnify the effects alluded to above. Following this line of reasoning, it has been argued that virus concentration in genital secretions may also increase over

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