How is HIV spread during anal sex ?

Unprotected anal sex with a person who has HIV or whose HIV status you do not know is the highest-risk sexual activity for both men and women. The walls of the anus and rectum are thin and have many blood vessels that can be injured during anal sex. HIV-infected semen can be easily absorbed through these thin walls and into the bloodstream. Injured tissue in the anus and rectum can expose the penis to blood containing HIV.

Using latex condoms for anal sex lowers HIV risk, but condoms fail more often during anal sex than during vaginal or oral sex. So, protected anal sex is still riskier than protected vaginal or oral sex.