Bladder fistulas develop when trauma, surgery, radiation therapy, inflammation, or infection damages the bladder wall enough to create an abnormal opening. The most common cause is surgery, particularly hysterectomy. More than half of all fistulas develop as a result of this procedure. Diverticulitis, an inflammatory disease of the intestines, is another common cause. The inflammation can weaken the intestine wall until an abscess develops and breaks open. Cancer is also a cause of bladder fistulas, but indirectly: the radiation therapy associated with cancer treatment can weaken the organ walls and cause a fistula to open.