
An anal fistula usually starts with an infection that arises from the anal glands in the anal canal. This infection then tracks towards the buttock skin and form a swollen pocket of infected tissue and liquid (an abscess). This is known as a perianal or ischiorectal abscess.
The abscess may either spontaneously burst and discharge pus. If it doesn't, a patient will have to undergo surgery to drain it. In either scenario, some of the patients will have the original channel persisting and staying open, and this eventually forms a permanent pathological channel between the anal canal and the skin.
In a minority of the cases, anal fistulas arise as a result of an underlying condition called Crohn's Disease. Patients with Crohn's disease can have multiple fistulas that do not heal.