Many STDs are transmitted more easily from man to woman than from woman to man (Harlap et al., 1991). For example, the risk to a woman of acquiring gonorrhea from a single act of intercourse may be as high as 60 to 90 percent, while transmission from a woman to man is about 20 to 30 percent (Holmes et al., 1970; Hooper et al., 1978; Platt et al., 1983; Judson, 1990; Donegan et al., 1994). Among couples where only one partner was initially infected, the annual risk of transmission of herpes simplex virus was 19 percent from man to woman, but only 5 percent from woman to man (Mertz et al., 1992). The comparative efficiency of male-to-female versus female-to-male transmission of HIV seems to differ according to the study population (Haverkos et al., 1992). Studies in the United States (Peterman et al., 1988; Padian et al., 1991) generally have shown greater efficiency of transmission from man to woman than from woman to man, while studies in Haiti (Deschamps et al., 1996) and Europe (de Vincenzi, 1994) have shown no significant gender difference in efficiency of transmission.