A fistula is defined as the connection of two body cavities or as the connection of a body cavity to the skin (such as the rectum to the skin). This is not a normal occurrence. One way a fistula may form is from an abscess (pus in the body). The abscess may be constantly filling with body fluids such as stool or urine, which prevents healing. Eventually the fistula breaks through to the skin, another body cavity, or an organ. An anal abscess is an infected cavity filled with pus found near the anus or rectum. If the skin over the abscess breaks, pus may discharge out. Many people may confuse this with hemorrhoids (piles). An anal fistula, almost always the result of a previous abscess, is a small tunnel connecting the anal gland from which the abscess arose to the skin of the buttocks outside the anus. Causes: An anal fistula usually develops after an anal abscess (a collection of pus) bursts or when it has not been completely treated. A fistula can also be caused by Tuberculosis and conditions that affect the intestines, such as Crohn's disease or ulcerative colitis.