An anal fistula is a small tunnel that forms between the inside of the anus and the nearby skin. It usually starts after an abscess bursts or heals poorly, and it keeps leaking like a broken pipe.

Ongoing drainage is the hallmark. You notice yellow, green, or bloody streaks on your underwear that return every day, even after a shower.

Pain comes and goes. It feels like a deep, dull throb during bowel movements and eases when the tunnel drains.

Swelling pops up. A pea-sized lump near the an opening opens and drains, then shrinks, only to swell again.

Itching and skin irritation are common. The constant moisture burns the skin around the anus, so you scratch without thinking.

Fever is rare, but a new painful lump or red streaks means the infection is spreading.

Late signs include multiple openings or a large, firm mass—an alarm that the fistula has branched.