CDC Fact Sheet (Information supplied by Regional Resource Network Program/US Dept of Health and Human Services)
The risk of getting HIV varies widely depending on the type of exposure. Some exposures, such as exposure to HIV during a blood transfusion, carry a much higher risk of transmission than other exposures, such as oral sex. For some exposures, risk of transmission, while biologically plausible, is so low that it is not possible to provide a precise number.