The anus is the opening through which the terminal portion of the large intestine (colon) joins the outside of the body. It plays many important roles, among which, expulsion of feces. The anus contains a group of muscular fibers called “sphincter ani externus”, which means external anal sphincter. The sphincter is endowed with a group of cells that allows the anal cavity to open during defecation (expulsion of feces from inside the rectum), and remain closed during normal times (periods of non defecation).
Anal cancer is due to genetic mutation of cells that line the cavity and surface of the anus. That is, you have anal cancer when healthy cells in your anal cavity turn into abnormal cells and multiply chaotically. The cause of this abnormal cellular proliferation is not well known to scientists; however, some risk factors are suspected (see risk factors):