Episode 33 of "Futurama"
s03e04 Parasites Lost

When Fry eats an egg-salad sandwich from a vending machine at a gas station, he begins to have strange side-effects -- he becomes stronger and smarter. Professor Farnsworth makes a diagnosis and concludes that Fry had ingested "intelligent worms" that have set up shop in his body. Because these worms are so smart, the regular means of flushing them out will not work. The Planet Express crew must shrink themselves to microscopic form and enter Fry's body to fight off the intruders. Meanwhile Leela is left to divert Fry's attention, but finds herself more and more attracted to the new man Fry has become.