Cancer of the anus is rare. It is slightly more common in women than in men. As with most cancers the cause of anal cancer is unknown. It is known that anal cancer is more likely to develop in people who have had a particular virus infection called the human papilloma virus (HPV). The risk of having HPV increases with the number of sexual partners you have. Anal cancer is also more common in people who have a lowered immunity due to medical conditions, such as HIV. Cigarette smoking may also increase a person's risk of developing anal cancer.