We could give you numbers, but they wouldn't mean much. As one AIDS expert pointed out in an online discussion group, "The risk of HIV transmission is not like the risk of losing at the races. Because you can't recoup the loss represented by infection, you can't think of the odds in the same way." Instead, let's talk about high-risk activities. People most at risk of acquiring HIV, the virus that leads to AIDS, are those who have condomless anal sex with an infected person, and people who share a needle with an infected person while injecting drugs. In the U.S., those groups account for the vast majority of AIDS deaths.

  There is also risk if you have unprotected vaginal or oral sex with an infected person. HIV is spread sexually when the blood or semen of an infected person makes contact with a cut or sore or mucous membrane of a partner. The use of condoms and other barriers should prevent this. Since it's impossible to tell if a partner has HIV (people lie), even if you've been together a long time (people cheat), you have to make certain uncomfortable assumptions. At the same time, no one wants to have sex with plastic for the rest of their life, and people take what they consider acceptable risks.