Abnormal connection between two hollow organs (e.g. bladder and intestine), or between a hollow organ and the skin (as in anal fistula). Fistulas can occur for no clear reason, but some conditions are known for a tendency to fistula formation, particularly intestinal cancers, diverticulitis, Crohn’s disease and intestinal tuberculosis. The symptoms of these disorders predominate, and effects of the fistula depend on the nature of the connection which it forms. Fistulas between bladder and intestine can cause faeces in the urine, and vice-versa. A fistula from intestine to vagina can introduce pus or contents of the intestine into the vagina. Anal fistulas usually exude pus on to the skin. Internal fistulas are hard to find, and tests for them should always be subordinate to tests to establish the primary cause; X-rays with contrast medium may reveal the fistula. External fistulas are slow to heal. Treatment is by surgical removal; recovery can be slow and fistulas tend to recur.