Many sexual infections, including HIV, gonorrhoea, syphilis, Hepatitis B and C and chlamydia can be present in semen and inside the anus. Herpes sores and genital warts can form inside the anus where they might not be noticed. Click here to read more about these sexually transmitted infections (STIs).
Because the anus is not so well lubricated as the mouth or vagina, the skin there can break during sex. This can allow infected semen directly into the blood stream. The delicate skin around the head of the penis can also break during sex and allow infection from the anus into the blood stream. This is why, for anal sex, it’s important to use lots of water-based lubrication as well as condoms.