Fistulas can arise in any part of the body, but they are most common in the digestive tract. They can also develop between blood vessels and in the urinary, reproductive, and lymphatic systems. Fistulas can occur at any age or can be present at birth (congenital). Some are life-threatening, others cause discomfort, while still others are benign and go undetected or cause few symptoms. Diabetics, individuals with compromised immune systems (AIDS, cancer) and individuals with certain gastrointestinal diseases (Crohn's disease, inflammatory bowel disease) are at increased risk of developing fistulas.

Fistulas are categorized by the number of openings they have and whether they connect two internal organs or open through the skin. There are four common types:

• Blind fistulas are open on one end only.
• Complete fistulas have one internal opening and one opening on the skin.
• Horseshoe fistulas are complex fistulas with more than one opening on the exterior of the body.
• Incomplete fistulas are tubes of skin that are open on the outside but closed on the inside and do not connect to any internal structure.

Anal and rectal fistulas develop in the wall of the anus or rectum. They connect the interior of the body to one or several openings in the skin. Anal and rectal fistulas almost always begin as an inflammation in an anal gland. The inflammation then moves into muscle tissue and develops into an abscess. In about half of all cases, the abscess develops into a fistula, degrading the muscle until an opening in the skin is created. About 9 people of every 100,000 develop anal fistulas, with men almost twice more likely to develop the condition than women. Although they may develop at any age, the average age for the development of anal fistulas is 38.

Intestinal fistulas can develop in both the large and small intestine. They are commonly associated with diseases such as inflammatory bowel disease (IBD) and Crohn's disease.

Tracheoesophageal fistulas (TEF) are usually birth defects. The windpipe, or trachea, is abnormally connected to the esophagus. This allows air to enter the digestive system and makes it possible to breathe food into the lungs (aspiration). In many cases, the esophagus is also incomplete, causing immediate feeding problems. There are several types of TEFs categorized by where the fistula is located and how the esophagus and trachea are connected, but all are life-threatening and require prompt surgery to repair. TEFs occur in about one of every 1,500–3,000 births.

The most common type of fistula involving these systems is a vesicovaginal fistula, in which the woman's vagina is connected to the urinary bladder. This causes leakage of urine from the vagina and results in frequent vaginal and bladder infections. Fistulas may also develop between the vagina and the large intestine (a enterovaginal fistula) so that feces leaks from the vagina. Although both these types of fistulas are uncommon in the developed world, they are common in poor developing countries and result from long, difficult labor and childbirth, especially in very young girls. As a result, they are sometimes referred to as obstetric fistulas.

Some experts suggest that in parts of Africa, as many as 3–4 women develop these fistulas out of every 1,000 births. Others estimate that as many as 2 million women worldwide are living with unrepaired obstetric fistulas. If left unrepaired, obstetric fistulas cause women to constantly leak urine and feces. As a result, they become social outcasts, causing them extreme hardship and psychological trauma.

Arteriovenous fistulas (AVF) can develop between an artery and a vein in any part of the body. These fistulas vary in size, length, and frequency. Arteries contain blood carrying oxygen to all parts of the body, while veins carry blood that has given up its oxygen back to the lungs. Connections between arteries and veins cause changes in blood pressure that result in abnormal development of the walls of the arteries and abnormal blood flow. Arteriovenous fistulas that are present at birth are sometimes referred to as arteriovenous malformations (AVMs). Many arteriovenous fistulas are present, but not evident at birth, and become obvious only after trauma. AVFs can also be acquired from penetrating trauma.

The causes and symptoms of fistulas vary depending on their location. Anal and rectal fistulas are usually caused by an abscess. Symptoms include constant throbbing pain and swelling in the rectal area. Pus is sometimes visible draining from the fistula opening on the skin. Many individuals have a fever resulting from the infection causing the abscess.

Vaginal fistulas are caused by infection and trauma to the tissue during childbirth. They are easily detected, because the woman smells unpleasant and leaks urine or feces through her vagina. Rarely these fistulas may develop as a complication of hysterectomy.

Tracheoesophageal fistulas are the result of errors in the development of the fetus. They are evident at birth, because the infant is unable to swallow or eat normally and are considered a medical emergency that requires surgery if the infant is to survive.

Arteriovenous fistulas are most often congenital defects. Symptoms vary depending on the size and location of the fistula. Often the skin is bright pink or dark red in the area of the fistula. Individuals may complain of pain. The pain is a result of some tissues not receiving enough oxygen because of abnormal blood flow.

Tests use to determine the presence of a fistula vary with the location of the fistula. When there is an opening to the outside, the physician may be able to see the fistula and probe it. Various imaging studies such as x rays, CT scans, barium enemas, endoscopy, and ultrasonography are used to locate less visible fistulas.

Anal and rectal fistulas are treated by draining the pus the infected area. The individual also is usually given antibiotics to help prevent recurrence of the abscess. If this fails to heal the fistula, surgery may be necessary.

Intestinal fistulas are treated first by reducing the inflammation in the intestine and then, if necessary with surgery. Treatment varies considerably depending on the degree of severity of symptoms the fistula causes. TEFs are always treated with surgery. Obstetric fistulas must also be repaired with surgery. The treatment of arteriovenous fistulas depends on the size and location of the fistula and usually includes surgery.

No effective alternative treatments for fistulas are known.

The outcome of fistulas depends on the type and cause of the condition. Surgical repair of obstetric fistulas is almost always successful. Unfortunately, many women in developing countries do not have access to this type of surgery. Treatment of anal and rectal fistulas is almost always successful, although fistulas may recur in up to 18% of individuals. The outcome of surgery on TEFs is highly variable, especially since infants born with this condition often have other developmental abnormalities that may affect the outcome of fistula repair. The degree of successful repair of arteriovenous fistulas depends on their size and location. Uncontrolled bleeding is the most common complication of surgery to repair AVFs.

Obstetric fistulas are the only preventable fistulas. These can be prevented with good prenatal and childbirth care and by avoiding pregnancy in very young girls. Although anal and rectal fistulas are not preventable, their damage can be minimized by prompt drainage and treatment.