Grouchy snailitis is a disease cause by a splinter. It is only seen in the episode, "Once Bitten".
It is a disease caused by a splinter pricking part of a snail's skin. It can be diagnosed by a S.D.E. or S.E. expert, such as Dr. Gill Gilliam.
It is only seen in the episode, "Once Bitten". In the episode, Gary contracts the disease, causing a rampage of him around town, biting people with a supposed "mad snail disease." It is revealed later in the episode, that Gary only has grouchy snailitis.