The Grim Adventures of Billy and Mandy - Season 1 - Eps 18: Billy's Growth Spurt
Billy eats too much junk food that makes him feel sick. Grim prepares him a remedy using his granny's recipe, but this medicine makes a parasite to start growing in Billy's back, which Billy names Yap-yap. In the following, Billy and Yap-yap continue eating junk food, so Mandy suggest to use the antidote to the original remedy. Eventually, Yap-yap, now looking like a smaller version of Billy, detaches himself from Billy (which would have technically cured Billy without the need of him drinking the anitdote), and tries to escape from the antidote. In the end, Yap-yap drinks the antidote and is destroyed.
About The Grim Adventures of Billy and Mandy

Title: The Grim Adventures of Billy and Mandy
First Air Date: 2001-08-24
Last Air Date: 2007-11-09
Status: Ended
Rating: 8.051/10 (from 964 votes)
Language: EN
Seasons: 8
Total Episodes: 184
Network: Cartoon Network
Genres: Animation, Comedy, Kids
Production Companies: Cartoon Network Studios
Synopsis
The exploits of the Grim Reaper, who has been forced into being the best friend of two children. A spin-off of the show Grim & Evil.
Cast

Greg Eagles
Grim (voice)

Richard Steven Horvitz
Billy (voice)

Grey DeLisle
Mandy (voice)

Vanessa Marshall
Irwin (voice)