You can become HIV-positive if you are exposed to the blood, semen, or vaginal fluid from someone who is HIV-positive and who is not virally suppressed. If someone who is HIV-positive is on treatment and are virally suppressed, then the risk of being infected by HIV is dramatically lower.
Being exposed to HIV can occur through unprotected sex and/or through sharing a needle. Anal sex without a condom is a very risky type of unprotected sexual behaviour and can easily lead to becoming infected with HIV or passing it on to somebody else. The risk of HIV transmission through anal sex without a condom and waterbased lubricant is about 18 times higher than for vaginal sex.
The risk of getting infected with HIV is much higher for the bottom (receptive) partner than the top (penetrating) partner. This is because the bottom guy is more likely to get semen inside his anus, which is the easiest way to get HIV.
