Fistula is a boss that can appear:
The large Fistula has 60 HP, the medium forms have 15 HP, and the small forms have 8 HP.
- Moves around the room slowly, bouncing off the walls.
- When enough health is depleted, the one large Fistula breaks into four medium Fistulas.
- Medium forms behave identically but move slightly faster and can bounce off other enemies.
- Each of the four medium Fistulas breaks into two small Fistulas.
- Small forms behave identically but move very fast.
- Each of the smallest Fistulas spawns a Charger with 15 health points on defeat unless they are over a pit or other obstacle.
All stages of Fistula deal a whole heart of contact damage.
Champion Versions 
- Dark Grey: 15% larger with 25% more HP (75). Spawns Boom Flies with 15 health points instead of Chargers. Each piece travels much slower and splits into three medium Fistulas as opposed to the original's four, similar to Teratoma.
- Fistula appeared in the original The Binding of Isaac.
- A fistula is an abnormal connection between organs, intestines, and other tissues.
- Fistula’s splittings after taking enough damage is a reference to the game Asteroid, where asteroids would float diagonally and split after taking enough damage. This was confirmed by Edmund McMillen.
- Fistula used to be a Caves/Catacombs boss in the original The Binding of Isaac, but as of Rebirth, it was a Cellar boss.
- It has been moved back into the Caves as of Repentance.
- Teratoma is the posthumous version of Fistula.
- Unlike the original The Binding of Isaac, Fistula doesn't bounce off of rocks.
- Fistula was inspired by a boss of the same name in another Edmund McMillen game, C-Word.
- Fistuloid is an enemy variant of this boss combined with what looks to be a Charger or Spitty.
- The Matriarch is a boss added in Booster Pack #5 that is simultaneously based of a Fistuloid, Chub, and this boss. The Matriarch will also spawn small Fistulas throughout the battle.
- Mother will spawn a unique Fistula with a rotten appearance during one of her attacks.
- A similar Looking Fistula can be encountered in the Corpse.
- Fistula is one of four bosses that can be spawned by It Lives, the other three being Chub, Polycephalus and Teratoma.
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