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Anal fistula is the medical term for an infected tunnel that develops between the skin and the muscular opening at the end of the digestive tract (anus).
Most anal fistulas are the result of an infection that starts in an anal gland. This infection results in an abscess that drains spontaneously or is drained surgically through the skin next to the anus. The fistula then forms a tunnel under the skin and connects with the infected gland.
Surgery is usually needed to treat anal fistula.