An anal fistula (also called a fistula-in-ano) is a small tunnel or communication channel that develops between the skin around and rectum anal fistula as a result of an infection or a collection of pus (abscess) in or around your anus. The anus is the external opening through which faeces are expelled from the body. Just inside the anus are a number of small glands. If one of these glands becomes blocked, an abscess or an infected cavity may form. An anal abscess is usually treated by surgical drainage, although some drain spontaneously. About 50% of these abscesses may develop into a fistula, in which a small tunnel connects the infected gland inside the anus to an opening on the skin around the anus. Anal Fistula can be higher or lower depending on whether and where they pass through the anal sphincter muscles.