Although both fistulas and sinus tracts are tubes formed after infection breaks down, they are not the same thing. Sinus tracts can form due to deep tissue infection caused by bacteria through the skin; trauma, such as some objects piercing the body; and infection caused by lymphatic tuberculosis. Anal fistula is a sequelae lesion of perianorectal abscess, and the underlying cause of perianorectal abscess is an infection of the anal glands. Why can’t a perianorectal abscess heal itself and form an anal fistula after it breaks down? It is generally believed that there are the following reasons: 1. Perianorectal abscesses break or incision is mostly outside the anus, and pus flows from the external mouth, but the primary infection hides in the anal sinus. The anal sinus is the gateway to secondary infection, because the anal sinus opens upward and opens in the rectal cavity, bacteria and intestinal contents can enter the abscess cavity through the anal sinus, causing repeated infections and forming fistulas. 2, fistula pass between the anal sphincter, due to the frequent contraction and diastole of the sphincter, affecting the pus discharge, easy to store pus infection and difficult to heal. 3, after the abscess breaks, the pus is discharged, the pus cavity gradually shrinks and the cavity wall forms connective tissue hyperplasia of the silhouette canal wall and can not heal. 4, fistula bending, or branching, poor drainage, repeated infection, resulting in fistula does not heal.