Actually, the incidence of HIV seroconversion is roughly 1:300. I know this because this is the risk medical professionals are at when dealing with accidental needle sticks. If you are directly injecting into veins, I'd imagine the risk is even higher.
That makes more sense. I recall reading an article written by an AIDS expert that the odds of a male contracting HIV from vaginal intercourse through any one unprotected encounter with a female confirmed to be HIV poz was something like 1/1100. I don't remember what the article said the odds were for needle sticks, needle sharing, etc - but the odds for those incidents were much higher than the female to male vaginal transmission.
