A fistula is an unusual connection between two empty spaces (specifically, two epithelialized surfaces), Like: blood vessels, intestines, or other unfilled (empty) organs. Fistulas are generally, caused by injury or surgery, but they can also effect from an infection or inflammation.  Fistulas are usually a disease condition, but they may be surgically created for therapeutic reasons.

Ultimately, it breaks through to the skin, other body cavity, or an organ, develops a fistula. Fistulas are more usual in Crohn's disease than they are in ulcerative colitis.

Types:

Fistulas usually happen in the area around the genitals and anus (known as the perineum). 

The four types of fistulas are:

Enterocutaneous: It's a type of fistula is from the intestine to the skin. An enterocutaneous fistula may be a difficulty of surgery.  It can be defined as a passageway that processes from the intestine to the surgery site and then to the skin.

Enteroenteric or Enterocolic: This is a fistula that affects the large or small intestine.

Enterovaginal: This is a fistula that passes to the vagina.

Enterovesicular: This type of fistula passes to the bladder. These fistulas may result generally urinary tract infections or the passage of gas from the urethra throughout the urination.