Researchers continue to investigate the causes of anal cancer. Known risk factors that have been identified include chronic infection with high-risk types of human papillomavirus HPV , gender and age. HPV, a collection of more than viruses spread primarily by contact during vaginal, oral or anal sexual activity, is responsible for the majority of anal cancers. HPV may be asymptomatic for years, and persistent HPV infections may cause cell changes that, left untreated, may become cancer. The HPV vaccine provides protection against numerous strains of the virus, including subtypes 16 and 18, which are responsible for most HPV-related cancers. Anal intercourse: Men and women with a history of receptive anal intercourse, especially those younger than 30, have a higher risk of anal cancer.