An anal fistula is an abnormal passage between the skin around the anus and the rectum, with an external opening around the anus and an internal opening in the rectum. When pus accumulates in the fistula, it flows out through the external opening, resulting in recurrent local swelling and pain as well as pus flow. The fistula is not self-healing, and eventually requires surgery to cure it completely, which requires a complete incision of the fistula and the internal opening to allow it to heal again.