A fistula may form when an abscess (a pocket of pus in the body) does not heal and eventually breaks through to the skin, another body cavity, or an organ. Fistulas are more common with Crohn's disease than with ulcerative colitis; about 25 percent of people with Crohn's disease develop a fistula.
Crohn's and Colitis Foundation of America. "Surgery for Crohn's Disease." CCFA Mar 2006. 17 Jul 2007.