males, as they also do in developed regions. The ratio of female to male prevalence generally rose in 2000-2001 worldwide (the major exception being Eastern Europe and Central Asia).

Women, particularly girls and young women, face a higher risk of infection with HIV for physiological, social, and cultural reasons. The risk of infection during unprotected sex is two to four times higher for women than for men (UNFPA, 2003). There are several reasons for this disparity; the viral load is generally higher in semen than in vaginal secretions; in vaginal intercourse a larger surface area is exposed to sexual secretions for a woman than for a man; and the vagina and cervix of adolescent women are less mature, with a thinner cell structure that allows the virus to pass more easily (Berman and Hein, 1999; Watstein and Laurich, 1991).

Variation in the female-male ratio across countries among those infected is also related to differences in patterns of heterosexual relations, such as differences in the number of partners, age differences between partners, and the use of commercial sex workers, as well as to variations in the importance of other modes of HIV transmission. In sub-Saharan Africa, for example, where most infections are transmitted by heterosexual relations, young women generally face higher risks because they tend to have sex with and marry older men, who are more likely to be infected than younger men. Some of these relationships are based on economic gain, that is, they involve the exchange of gifts or money for sex. There is now a great deal of research showing that the power differentials inherent in such relationships make it difficult for young women to negotiate the use of condoms (Blanc, 2001; Luke, 2003; Weiss, Whelan, and Gupta, 1996). Furthermore, in high HIV areas, early marriage does not protect young women from risk. While marriage reduces the number of sexual partners, it increases frequency of sex, decreases condom use, and virtually eliminates a girl’s ability to abstain from sex, except possibly during the postpartum period (Clark, 2004).

Sex between men accounts for at least 5-10 percent of HIV cases at all ages worldwide, and it is a predominant risk factor in some developing countries, including Brazil, Costa Rica, and Mexico (Summers, Kates, and Murphy, 2002). Young people comprise a large proportion of this population (Joint United Nations Programme on HIV/AIDS [UNAIDS], 1998, 2000a). Injecting drugs, again a particular problem among young men, accounted for over half of new cases in 1998-1999 in China, Malaysia, Russia, and Vietnam (Joint United Nations Programme on HIV/AIDS [UNAIDS], 2000b). This is a doubly disturbing trend because, in addition to the risks of transmission through injections, the use of drugs is also associated with an increased likelihood of engaging in risky sexual behavior (Summers, Kates, and Murphy, 2002).

The presence of certain other sexually transmitted infections greatly