The probability of contracting HIV is highest during a transfusion of infected blood, according to the Centers for Disease Control and Prevention. The risk per 10,000 exposures is 9,250. Receptive anal intercourse is the next most-likely way to contract HIV, where the risk is 138 per 10,000 exposures.
Needle sharing between drug users is the third most-likely method of HIV transmission, says the CDC. The risk per 10,000 exposures is 63.The risk involved in needle stick exposure is 23 per 10,000 exposures. The risk for insertive anal intercourse is 11 per 10,000 exposures, eight for receptive penile-vaginal intercourse and four for insertive penile-vaginal intercourse. The risk per 10,000 exposures for receptive and insertive oral intercourse is low, while the risks for biting, spitting, tossing bodily fluids at a person or sharing sex toys is negligible.
A woman who is infected with HIV can also pass the virus to her child during pregnancy, labor, delivery or breastfeeding, according to the World Health Organization. The rates for this type of transmission vary between 15 and 45 percent if the mother is not treated. With treatment, the rate can fall below 5 percent. The risk of transmission increases if the person already has a sexually transmitted disease, suffers from late-stage HIV infection or has high levels of the virus in his blood.