possibility that hormonal contraceptives may increase the likelihood of infectious genital tract secretions in HIV-infected women and/or increase susceptibility to HIV infection.

Cervical ectopy (extension of columnar epithelial cells present in the adult endocervix onto the exposed portion of the cervix within the vagina) has also been found to be a risk factor for HIV infection (Moss et al., 1991). Among women attending an STD clinic and among college women, cervical ectopy was positively associated with use of oral contraceptives and with chlamydial infection; ectopy disappeared with increasing age (Critchlow et al., 1995).

As previously discussed, other STDs can increase risk for acquiring or transmitting HIV infection. However, prior infection with certain STDs can provide specific immunity against reinfection with the same pathogen (Plummer et al., 1989; Brunham et al., 1994). Cross-immunity (protection conferred by prior infection with a different pathogen) also occurs. For example, a prospective study of women found that asymptomatic shedding of herpes simplex virus type 2 occurs more often during the first three months after acquisition of primary type 2 disease (Koelle et al., 1992). Among persons with herpes simplex virus type 2 infections, previous infection with type 1 virus was associated with a lower rate of asymptomatic viral shedding. This observation suggests that, as prevalence of herpes simplex virus type 1 infections in childhood decline, the risk of herpes simplex virus type 2 infection may be increased when this STD is encountered by a sexually active adult. Nonspecific immunity may make some individuals more resistant to certain STDs even though they have never experienced prior STDs or related infections. For example, the normal vaginal flora contains hydrogen-peroxidase-producing bacteria that have antimicrobial activity. Recent data suggest that women with bacterial vaginosis who lack hydrogen-peroxidase-producting bacteria (lactobacilli) are at increased risk of gonorrhea (Sharon Hillier and King Holmes, University of Washington, unpublished data, 1996; Martin et al., 1996).

Social Factors

On a population level, preventing the spread of STDs is difficult without addressing social issues that have a tremendous influence on transmission of STDs. Some fundamental societal problems such as poverty, lack of education, and social inequity indirectly increase the prevalence of STDs in certain populations. In addition, lack of openness and mixed messages regarding sexuality create obstacles to STD prevention for the entire population and contribute to the hidden nature of the STDs. In the following discussion, the committee highlights several social problems that directly affect the spread of STDs in subpopulations and shows how societal norms regarding sexuality impede prevention of STDs. In Chapter 4 the committee describes interventions that can be used to lessen the

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