that place others at risk for HIV infection and place themselves at risk for other STIs. Still other evidence (Parsons et al., 2005) suggests that some HIV-positive men who have sex with men appear to engage more in harm reduction strategies—for example, serosorting (selecting sexual partners based on HIV serostatus) and strategic positioning (assuming the receptive or insertive position during sexual intercourse according to HIV serostatus, with the receptive position posing the greater risk of transmission)—than in withdrawal before ejaculation during anal intercourse.

While studies with racial and ethnic minority men are urgently needed, strong evidence from a meta-analysis suggests that individual-, group-, and community-level behavioral interventions are effective in reducing the risk of acquiring sexually transmitted HIV in adult men who have sex with men (Herbst et al., 2007). As this analysis shows, and the social ecology model predicts, there are multiple levels at which interventions can have an impact on individual behavior.

Far less research on HIV has been conducted with women who have sex with women. According to Lesbian Health (IOM, 1999), prevalence rates of HIV among women who have sex with both women and men were higher than those among exclusively heterosexual or exclusively homosexual women. More recently, it has been noted that while female-to-female transmission of HIV appears to be possible, there have been no confirmed cases (CDC, 2008).

Although HIV has overwhelmed the field of STIs, there are other STIs of concern. In 2009, 42 clinics at 12 sites across the United States submitted STI and HIV data to the Centers for Disease Control and Prevention (CDC). Results from these clinical sites showed a median prevalence of 14.9 percent for gonorrhea overall among men who have sex with men (with a range of 6.5 to 27.9 percent). The median prevalence of chlamydia among men who have sex with men was 11.2 percent (with a range of 4.5 to 18.5 percent). Primary and secondary syphilis increased between 2005 and 2009, with men who have sex with men accounting for 62 percent of all primary and secondary syphilis cases in the United States (CDC, 2010). Median clinic syphilis seroreactivity (used as a proxy for syphilis prevalence) among men who have sex with men tested for syphilis increased from 4 percent in 1999 (range of 3 to 13 percent) to 11 percent in 2008 (range of 8 to 17 percent) (CDC, 2009b). STI (including HIV) positivity varied by race and ethnicity but tended to be highest among black men who have sex with men. Median positivity for STIs was higher among HIV-positive than among HIV-negative men who have sex with men (CDC, 2009b). In a recent study of 212 HIV-infected men who have sex with men, the baseline prevalence of asymptomatic STIs was found to be 14 percent (Rieg et al., 2008). Studies also suggest that gonorrhea and syphilis rates have been



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