Mad Snail Disease is a mythological disease/virus that was believed that snails could be afflicted with. It appears in the episode "Once Bitten" and the books Mistaken Identity and Attack of the Zombies!
Patrick explains to Squidward about its supposed existence before he and everyone else are informed by Dr. Gill Gilliam that it is a myth. The virus causes massive panic within Bikini Bottom, as it is feared that a bloodborne and later an airborne outbreak had started in the town. MSD was believed to be a highly contagious virus that caused rage in snails and zombie-like symptoms in fish and other creatures. This leads to mass hysteria and destruction throughout the town as residents were "infected" and turned into "zombies."
Squidward gets bitten by Gary and "infected" with Mad Snail Disease. When Patrick sees Squidward getting bitten by Gary, he tells Squidward that Gary has "Mad Snail Disease." SpongeBob tries to take Gary to Snail Care, but Gary escapes and bites nearly every Bikini Bottomite, supposedly starting a rage-filled outbreak of MSD.
As the last "survivors" of the "outbreak" watch in horror as the raging Gary bites SpongeBob, supposedly infecting him, Dr. Gill Gilliam tells the Bikini Bottomites that Mad Snail Disease is just an old urban legend and does not exist. He reveals that pain and irritation simply caused Gary's rabies-like symptoms from a splinter in his foot from Squidward's anti-snail defense. Gary was not infected, and he did not start an outbreak of anything except for mass hysteria. The only thing inflicting Gary was Grouchy Snailitis.
Just as the "infected" broke into the fortified Krusty Krab, Squidward continues to groan like a zombie, albeit admitting he was not, and that he was groaning because he had to go to work.
- Ticklish ribs
- Loss of balance
- Bloodshot eyes
- Sore throat
- Messy pants
- Severely untrimmed toenails
- Large rash
- Flesh-craving zombification
- It appears to cause symptoms similar to rabies. However, this seems to be exclusive to snails such as Gary, who was the source of the "outbreak."
- It is a parody of Mad Cow Disease.