Fistula

An anal fistula is usually a side effect of an anal abscess, an infected wound that drains pus from your anus. The draining abscess can create a tunnel through your anus to the skin outside. Anal pain, swelling and redness are the primary symptoms.

Fistulas can occur throughout your body, either between different organs or from an organ to an opening in your skin. They usually occur when your tissues are inflamed for a long time, due to an injury or disease. Chronic inflammation and infection can eventually erode into the nearby tissues, especially when pus needs to drain.