Anal intercourse

Unprotected (i.e. condomless) anal intercourse is a highly efficient way of transmitting HIV, and it is considered a high-risk activity for both partners, although the exact degree of risk can depend on many factors. For each unprotected act with an HIV-positive partner, the risk of infection has been estimated at 0.82% and 0.06% for the receptive and insertive partners respectively,¹ but may be ten to 25 times higher if the positive partner is recently infected.

The receptive or 'passive' partner is at risk of infection from HIV in the semen and pre-seminal fluids ('pre-cum') of the infected partner. Rectal tissue is delicate and easily damaged, which can give the virus direct access to the bloodstream. However, such tissue damage is not necessary for infection to occur: the rectal tissue itself is rich in cells which are directly susceptible to infection.

The insertive or 'active' partner is also at risk of infection, as high levels of HIV can be present in secretions and blood from the rectal tissues. This creates a risk of transmission to the insertive partner through the tissue in the urethra and on the head of the penis – particularly underneath the foreskin.

Studies have identified a number of factors that increase the risk of transmission even further, including sexually transmitted infections and poppers use; these are discussed below, under Co-factors affecting risks. However, unprotected anal intercourse remains a high-risk activity even without these additional risk factors.