Infection by the human papillomavirus (HPV) is the most important risk factor for anal cancer. Most squamous cell anal cancers are linked to infection with HPV. HPV is a group of more than 150 related viruses, the same group of viruses that causes cervical cancer, as well as other kinds of cancer. In fact, women with a history of cervical cancer (or pre-cancer) have an increased risk of anal cancer.

Kind of wild that the linked article fails to mention STIs and HPV.