- Squidward Tentacles
- Gary the Snail
- SpongeBob SquarePants
- Patrick Star
- Dave, Frank, Percy, Isabelle, Harold
- Frankie Billy
- Jimmy Gus
- Frank (red shirt)
- Nat Peterson
- Harold (red fish)
- Jennifer Millie
- Harold Bill Reginald Scott
- Harry (fish)
- Johnny Elaine
- Perch Perkins
- Evelyn Annette
- Martha Smith
- Miss Shell
- Nancy Suzy Fish
- Nathiel Waters
- Sheldon (fish)
- Abigail Marge
- Medley Fishbowl
- Monroe Timmy
- Ivy (gray)
- Tina Fran
- Vera Fishbowl
- Construction Foreman
- Officer Johnson
- Incidental 4
- Eugene H. Krabs
- Old Man Walker
- Susie Rechid
- Debbie Rechid
- Phil (yellow)
- Dr. Gill Gilliam (Name revealed)
- Shelly Fishbowl
- Other Bikini Bottomites
Squidward walks outside his house on a sunny day only to find out that his yard is covered in Gary's slime trail. He attempts to keep Gary away by building a splintery wooden fence around him, but to no avail. As he complains to SpongeBob about his pet, Gary bites him. Squidward asks SpongeBob if Gary has had all of his shots, and SpongeBob says that he has, until Patrick comes along and asks him if Gary has been vaccinated for "Mad Snail Disease," a disease that turns anyone bitten by the afflicted into a zombie, which SpongeBob says he has not.
Suddenly, Gary runs off and starts to bite everybody in sight. Bikini Bottom is sent into a state of mass hysteria, and destruction, and soon, almost everybody has been bitten and turned into zombies. Those who have survived the disease are hiding out at the Krusty Krab. SpongeBob tries to get in, but Patrick believes he is a zombie and will not let him in until he slips through the doors. Old Man Jenkins suggests that they might all be zombies, and everyone starts panicking. Gary then comes in, much to the horror of the onlookers, and SpongeBob tries to bring him to his senses, but he gets bitten, much to his horror. The survivors in the Krusty Krab then decide to terminate Gary.
However, a doctor named Gill Gilliam enters the building to put an end to the commotion, explaining that the disease is just an urban legend and that the supposed symptoms were all in their imaginations, and that Gary was simply suffering from "Grouchy Snailitis" due to a "tiny" splinter in his foot from Squidward's fence, which he removes. SpongeBob is happy that Gary was never disease-ridden and asks if he still loves him with Gary meowing in response. The remaining people outside appear to hear this, and even though they are not infected, Squidward still acts like a zombie due to the fact that he is now working at his job and starts serving customers. The episode ends with Gary biting Mr. Krabs for no reason at all.
|( ‣ ) Associated production music|
|( • ) Original music|
|( ◦ ) SpongeBob music|
‣ Marching to Honolulu - Kapono Beamer [Title card]
‣ Comic Walk - Sidney Torch ["Snail trail..."]
‣ Toyland March 2 - Harry Bluestone, Emil Cadkin [Squidward putting up a wooden fence]
‣ Tomfoolery - David Snell ["This cheap splintery wood will keep even Gary out."]
• Seaweed - Steve Belfer [Squidward yells at SpongeBob]
‣ Lap Steel - Nicolas Carr ["Gary!"]
‣ Lighting the Fuse (a) - Dick Stephen Walter [Squidward asks if Gary's had all his shots]
‣ Lighting the Fuse (b) - Dick Stephen Walter [Squidward asks if Gary's had all his shots]
‣ Comic Tension (a) - Dick Stephen Walter [Squidward naming snail diseases]
‣ Twilight Zone - Harry Bluestone, Emil Cadkin [Patrick describing Mad Snail Disease]
‣ Shock Entrance [#24] - Harry Bluestone, Emil Cadkin [Patrick talking Squidward about Mad Snail Disease's symptoms]
‣ Twilight Zone - Harry Bluestone, Emil Cadkin ["...will turn you into... A zombie."]
‣ Dramatic Impact 3 - Ivor Slaney [Gary's missing]
‣ House of Horror - William Farran [Patrick warns people about Mad Snail Disease]
‣ Lambs in Clover - Jack Strachey ["Oh, look, honey, isn't he the cutest?"]
‣ Tales from the Swamp (a) - Ron Goodwin ["Mad snail disease is real!"/Everyone panicking]
‣ Bikini Bottom News Theme - Nicolas Carr [news blast]
‣ Cellar Search - Philippe Pares [Squidward interviewed]
‣ Robot March - Harry Bluestone, Emil Cadkin [Second scene of everyone panicking]
‣ March of the Aliens - Harry Bluestone, Emil Cadkin [zombies chasing SpongeBob/SpongeBob runs to the Krusty Krab]
‣ The Tip Top Polka/The Cliff Polka - Chelmsford Folk Band ["SpongeBob! Come in, boy."]
‣ Search Party 3 [#51] - Harry Bluestone, Emil Cadkin ["Stop! You can't let anyone in!"]
‣ Lap Steel - Nicolas Carr [SpongeBob squeezes through the door crack]
‣ Vibe Q Sting - Nicolas Carr ["Or is it?"]
‣ Search Party 1 [#48] - Harry Bluestone, Emil Cadkin ["I don't believe that's the real SpongeBob."]
‣ Vibe Q Sting - Nicolas Carr [Patrick thinks for a second]
‣ Death of the Alien 1 [#38] - Harry Bluestone, Emil Cadkin [Everyone panics/Gary appears at the Krusty Krab]
• Steel Licks 19 - Jeremy Wakefield ["All those people think you're a monster."]
‣ Death of the Alien 1 [#38] - Harry Bluestone, Emil Cadkin [Gary bites SpongeBob]
‣ Wooden Bear - Gil Flat, Tony Tape, Florian Voelxen [Mad Snail Disease doesn't exist]
‣ Old Hilo March - Kapono Beamer [Gary's back to normal]
‣ Cellar Search - Philippe Pares [Squidward enters]
• Steel Licks 6 - Jeremy Wakefield ["Oh, yes I am."]
‣ Old Hilo March - Kapono Beamer [ending]
- This episode is available on the Whale of a Birthday, Season 4 Volume 2, Complete Fourth Season, Triple Pack 1, Big Hits: Volume 1, Snail Meow, SpongeBob SquarePants Vol. 6, From the Beginning, Pt.2, The Third & Fourth Seasons, The Best 200 Episodes Ever, and First 100 Episodes DVDs.
- "Once Bitten" was ranked #39 during the Best Day Ever event on November 9–10, 2006.
- Viewers can easily tell there is no such thing as "Mad Snail Disease," because Gary had a splinter, and some symptoms Patrick told Squidward were not true because for loss of balance, Patrick knocked Squidward over and for ticklish rib cage, he just tickled Squidward himself.
- This episode marks the first time Perch Perkins appears in the show. This episode also marks the first time he is orange instead of purple. He remains that way until the second movie.
- This is the first of four episodes transcribed as a comic into the SpongeBob SquarePants/Tokyopop book Mistaken Identity. The other three were "That's No Lady," "The Thing," and "Rule of Dumb."
- This episode is one of the four episodes in season four to use the Crowd Scream sound effect, the others being "Selling Out," "Enemy In-Law," and "Dunces and Dragons" respectively.
- There's a book based on this episode called Attack of the Zombies!
- This episode premiered on Ian McShane's 64th birthday.
- The episode's title is based on the phrase, "Once Bitten, Twice Shy."
- Mad Snail Disease may be a reference to the real-life disease mad cow disease. However, the so-called "Mad snail disease" has the symptoms of rabies.
- During the news report with Perch Perkins, a fish states, "It's spreading through the air." This is impossible because they are underwater.
- When SpongeBob runs to the bus, the "Stop" sign is there, but in the next scene, it disappears.
- An elderly woman's dress turns from purple to green in the scene where the "zombies" chase SpongeBob.
- When the "zombies" attack the Krusty Krab, there is neither the Chum Bucket nor the path that leads to it.
- When SpongeBob says, "Gary, how could you?," his legs are missing.
- During the scene where SpongeBob says, "I knew you weren't disease-ridden!" to Gary, Frank's purple shirt sleeve turns blue. In the next scene, it changes back to purple.
- When SpongeBob arrives at the Krusty Krab, the doors are locked. Later on, when Squidward walks in acting like a zombie, the doors are unlocked.
- Nat is seen as a zombie, but later, he is later seen as a normal customer in the Krusty Krab.
- Pearl Krabs is listed in the credits for this episode. She was originally featured in a deleted scene, and Lori Alan recorded dialogue for it. After the scene was cut, Lori Alan's credit still remained.
- Bikini Bottomites panicking by Gary's infectious bite.