"HIV risk factor: sexual orientation
In the United States, the group with the highest rate of HIV infection is men who have sex with men (CDC, 2005). This group has the highest rates of unprotected anal sex, which is a behavioral risk factor for HIV transmission. In most regions of the world, however, the group with the highest rates of HIV is people who have heterosexual intercourse."

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http://www.apositivelife.com/forasos/demographic-risk-factors-for-hiv.html