"Parasites Lost" is the second episode in season three of Futurama. Although the title is a play on John Milton's epic poem Paradise Lost, the episode is a parody of the 1966 film Fantastic Voyage. It originally aired on the Fox network in the United States on January 21, 2001.

While making a pit stop at an interstellar truck stop, Fry buys and eats a decaying egg salad sandwich from a vending machine in the restroom. Upon returning to Earth, Fry and Bender are assigned the task of fixing the plasma fusion boiler, which promptly explodes. Bender is not damaged, but Fry is impaled by a large pipe. Despite the severity of the injury, Fry's damaged body repairs itself in seconds, and the subsequent medical examination reveals to the crew that Fry is infested with microscopic worms from the egg salad sandwich.

To eliminate the infestation, Professor Farnsworth makes miniature robotic versions of the crew, except for Fry and Leela. Because the worms know all that Fry knows and would thus defend themselves if Fry knew about the mission, Leela is assigned to distract Fry, who is not told what is happening. Controlling the micro-droids using virtual reality gear, the crew board a miniature Planet Express ship, and enter Fry's ear. Throughout the travel, the crew discover that the worms are drastically improving Fry's intelligence, health, and fitness.

The crew make their way into Fry's bowel, and fight their way to the pelvic splanchnic ganglion, intending to cause a massive bowel movement to expel the worm society. Meanwhile, Leela is enchanted by the now intelligent and muscular Fry. Fry reveals that he loves Leela but only recently was he able to articulate his thoughts. Leela realizes that the worms are responsible for the new, improved Fry, and sets out to stop the Professor. Her micro-droid reaches the nerve, and hacks the rest of the micro-droid crew to pieces with an axe. The Professor tells her that the worms will burrow so deep into Fry's body, he will be stuck with them forever, but Leela reasons that Fry is better off with the worms.

...
Wikipedia