Saletan examines the ban on gay men donating blood:
"Men who have had sex with men since 1977 have an HIV prevalence … 60 times higher than the general population," the [FDA}] observes. "Even taking into account that 75% of HIV infected men who have sex with men already know they are HIV positive and would be unlikely to donate blood," that leaves a population of MSM blood-donor applicants whose HIV prevalence is "over 15 fold higher than the general population."
So a 15-fold difference is good enough to warrant group exclusion. How about a nine-fold difference? According to the Centers for Disease Control, HIV prevalence is eight to nine times higher among blacks than among whites, and HIV incidence (the rate of new infections in a given year) is seven times higher. For black women, HIV prevalence is 18 times higher than for white women.