Fistula in Ano is a chronic inflammatory condition having a tubular structure, opening in Ano rectal canal at one end and surface of perianal skin on the other end.
Classical fistula has two openings. One internal (in Ano-reclalcanal) and other external (in the skin of peri anal area )
Chronic pus discharge in the peri anal area indicates – fistulous opening
Pain off and on, pus discharge from external opening some time it closes itself anal again opens after the pus formation
Deep seated abscess in the source of pus

CAUSES OF FISTULA:

An acute abscess in one or more potential spaces around Anorectad canal. Source of infection could be a number of predisposing conditions e.g.

Infection at the root of pile. mass
Infected and inflamed anal crypt.
Traumatized muco-cutaneous lining of Ano rectal canal due to indigested foreign body e.g. fish bone.
Inflamed or thromboseal condition of previously existing pile mass.
Retained sutures after Haemorrhoidectomy
Injection of chemical for treating piles
Foreign Body Penetrating from out side
Radiation burns from x-Rays and radiotherapy