An anal fistula is an abnormal tunnel or passage that forms between the inside of the anal canal and the skin around the anus. It usually develops as a result of an infection in the anal glands, which leads to the formation of an abscess. When the abscess does not heal completely, it can create a small channel that causes recurring discharge, pain, and discomfort.

Fistulas do not heal on their own and often require proper medical treatment to prevent repeated infections and complications.