A fistula is a medical condition in which an abnormal connection forms between two organs or vessels in the body. An ULCER is an open sore on the skin or mucous membrane, usually caused by infection or inflammation. In the case of a fistula, an ULCER is often present at the site of the abnormal connection. Fistulas can be caused by a variety of conditions, including Crohn's disease, diverticulitis, and radiation therapy. Treatment for a fistula typically involves antibiotics, surgery, or a combination of both.