Anal cancer is relatively uncommon compared to other cancers. In the general population only 1.5 men per 100000 and 1.9 women per 100000 will develop anal cancer. In HIV positive people the estimated rates are between 46-131 men per 100000 people, and 30 women per 100000. HIV positive men who have sex with men have the highest rates, and African American men are much more likely than other men to develop it.
Almost all cases are in people over 35 years of age.
It is still unknown why HIV positive people undergoing retroviral therapy have increased rates of anal cancer. Compared to many other cancers anal cancer is usually slow to develop so it is possible that the increased life span provided by retrovirals allows more time for the cancer to develop.