A fistula is a sequelae of a perianorectal abscess that breaks or is incised and fails to heal, eventually forming a fistula. The perianorectal abscess breaks or is incised mostly outside the anus, with pus flowing from the external opening, but the primary infection is mostly at the anal sinus. The anal sinus is then the gateway to secondary infections, i.e., the internal port, which is repeatedly infected and forms a fistula tract. The fistula passes between the anal sphincter, and because the sphincter is constantly contracting and diastolic, it compresses the fistula and affects the discharge of pus, and feces and gas from the rectum can constantly enter the fistula through the internal opening, which is curved, or has sinuses or branches that do not drain well, retaining pus and causing repeated infections, resulting in a fistula that does not heal easily. Anal fistula can only be cured by surgical removal of the fistula and the internal opening, and so far it cannot be cured by non-surgical methods. The principles of surgery are complete removal of the fistula and proper treatment of the internal opening.