... Maybe you fooled around with a guy 30 years ago and have spent the rest of your life as a celibate priest. Maybe you've been in a faithful same-sex marriage for 40 years. Maybe you've passed an HIV test. It doesn't matter. You can't give blood, because you're in the wrong "group." On the other hand, if you're in the right group—heterosexuals—you can give blood despite dangerous behavior. If you had sex with a prostitute, an IV drug user, and an HIV-positive opposite-sex partner 13 months ago, you're good to go. This kind of group-based screening is a long-standing...