Worms of Cologne

The Worms of Colón are sentient, mutualistic tapeworms that used to inhabit Fry's bowels. They entered Fry from an egg salad sandwich that he ate from a service station bathroom's vending machine, which contained the Worms' reproductive spores. Once in his body, the Worms immediately began to strengthen his body's systems, and led to Fry and Leela briefly being together. After the most of the Planet Express crew failed in their mission to evict the worms by irritating the pelvic splanchnic ganglion, Fry decided to get rid of them after feeling that Leela loved only what the Worms had made of him. Via a micro-droids, Fry forced the Worms to leave by threatening to cut his medulla oblongata, which would have killed him and the Worms.

Trivia

 * The Lord Mayor of Colón claims that they are what "makes special sauce so special."