An anal fistula is an abnormal connection between the lining on the inside of your anal canal (back passage) and the skin near your anus. 

Most anal fistulas are caused by an abscess (a collection of pus) which has developed in your anal canal (back passage). Just inside the anus there are small glands. When these glands get blocked, they can become infected and an abscess can develop. 

A fistula is a small tunnel that forms under the skin and connects a previously infected anal gland to the skin on the buttocks outside the anus. The pus can drain away onto the skin on its own or by an operation. A fistula happens when the track, made by the pus on the way to the surface of the skin, stays open. Surgery is usually needed to treat an anal fistula, sometimes preceeded by an endo anal ultrasound.