The Straight Dope: Update: What are your chances of getting HIV? To figure the combined risk of multiple behaviors, you multiply the relative risk numbers. In the table the baseline behavior (least risky by definition) is wearing a condom while getting fellated by someone who's tested negative for HIV. The riskiest behavior is being the bottom during unprotected anal sex with someone who's HIV positive. For gays this is 860,000 times more dangerous than their baseline, and for heterosexuals the risk increases by a factor of 9,412,000. That good old-fashioned blow job is looking better all the time