An anal fistula is an abnormal tunnel. It forms between the inside of the anal canal and the skin on the outside, near the anus. Most fistulas develop after an anal abscess bursts or gets drained. Once the abscess heals partially, a small track is often left behind. This track keeps discharging pus or fluid on and off. Since the tunnel has an opening on both ends, it rarely closes on its own. That is why surgery becomes necessary in almost all cases.