An anal fistula is a tract left after an anal abscess bursts spontaneously or is surgically drained, generally consisting of a primary internal opening, a fistula tract, and a secondary external opening. Thus, an anal fistula and an anal abscess represent two stages of the same disease: initially, there is an anal abscess, and then, after the pus from the abscess is drained, an anal fistula forms. Typically, the transformation from an anal abscess to an anal fistula takes about two to three months, meaning that an anal fistula can form about three months after the abscess bursts.