A fistula is an abnormal connection between an organ, vessel, or intestine and another structure. Fistulas are usually the result of trauma or surgery, but can also result from infection or inflammation.

Inflammatory bowel disease, such as ulcerative colitis or Crohn's disease, is an example of a disease which leads to fistulas between one loop of intestine and another (enetero-enteral fistula) or intestine and skin (enterocutaneous fistula). Trauma can lead to fistulas between arteries and veins (arteriovenous fistulas).

• Arteriovenous (between an artery and vein)
• Biliary (created during gallbladder surgery, connecting bile ducts to the surface of the skin)
• Cervical (either an abnormal opening into the cervix or in the neck)
• Craniosinus (between the intracranial space and a paranasal sinus)
• Enterovaginal (between the bowel and vagina)
• Fecal or anal (the feces is discharged through an opening other than the anus)
• Gastric (from the stomach to the surface of the skin)
• Metroperitoneal (between the uterus and peritoneal cavity)
• Pulmonary arteriovenous (in a lung, the pulmonary artery and vein are connected, allowing the blood to bypass the oxygenation process in the lung (pulmonary arteriovenous fistula)
• Umbilical (connection between the umbilicus and gut)

• Blind (open on one end only, but connects to two structures)
• Complete (has both external and internal openings)
• Horseshoe (connecting the anus to one or more points on the surface of the skin after going around the rectum)
• Incomplete (a tube from the skin that is closed on the inside and does not connect to any internal organ or structure)