Anal fistulas are small, abnormal tracts that run from the inside of the anus to the skin around the anus. They are usually the result of a blocked and infected anal gland. Instead of the infection breaking through back into the anus, it breaks through to the skin around the anus, thus creating a tunnel. As long as the internal opening remains open, the fistula usually will not heal. An anal fistula may declare itself after drainage of a previous anal abscess, and the wound does not heal properly. This could be because there was already an internal opening at the point, but it was not apparent when the abscess was being drained. As a result, there will be a persistent drainage of pus from the incision, and the wound will not heal and close completely. In other cases, the skin heals, but soon develops a swelling under the scar. When the scar bursts, pus will be discharged.