"Original Air Date: 1/21/01"
Fry buys an egg-salad sandwich from a gas station and starts to have strange side-effects. The Professor diagnoses him to have worms, but they aren't ordinary. They are actually improving Fry, making him smarter and stronger. Since the worms are too smart to be killed by regular methods, the crew shrinks themselves and they go inside Fry to destroy the worms. Leela stays behind so she can divert Fry. Leela starts to become more and more attracted to Fry so she goes inside Fry herself and stops the crew. Fry and Leela start to hit it off well, but then Fry starts to wonder if Leela likes the real Fry, or just the worms.
This is Episode #33 in the Futurama series. It is Episode #4 in Season 3.