Vomit Chain Reaction

And then Frank lost it, too, joining the chunk-blowing chorus, the barf brigade, five men in a tossing boat who might have reasons to kill each other, but who, for the moment, could think about nothing except when the next retch would come, and what it would bring up.

—Tricky Business

When one character barfs, soon everyone else starts barfing as well.

This is Truth in Television, since vomiting has been known to inspire vomiting, as part of a prehistoric instinct: if someone from your same tribe vomits because he ate something harmful, chances are you ate the same thing and therefore it's best if you hurl it too.

For the one person version, see Vomit Indiscretion Shot. Contrast Vomit Discretion Shot. A small version of Disaster Dominoes. A form of Brown Note.

Examples of Vomit Chain Reaction include:

Film

In The Goonies, Chunk says the worst thing he ever did was start one with fake vomit in a movie theater.
One of the first scenes in Scary Movie 2 is a parody of The Exorcist. It ends in a drawn out vomit fight between the girl, the priest, and his assistant.
A deleted scene in the third Austin Powers movie has Number Two vomiting upon witnessing Mr. Roboto getting eaten by the sharks, followed by everybody else in the room.
This happened in the movie Mafia! when a badly disfigured character disgustingly eats a piece of fruit, the chain reaction even reaches a horse. For extra bad-taste points, this all occurs at a funeral.
In the second Problem Child movie, Junior causes a massive one of these by playing with the speed controls for a theme park ride and thus causing everyone on it to get motion sick.
The Mr Creosote scene in Monty Python's The Meaning of Life. Fun fact: one of the extras? Not faking it.
In the film The Hairy Bird, after the boys get drunk, they are given ipecac to sober up for their performance. As they settle into the stands, it starts with just one boy getting sick, and the rest follow. The girls run from the stage.
The story of Lardass in Stand by Me has him trigger this on purpose for revenge.
In Porkys 2 one of the girls simulates vomiting into a decorative pool in a very fancy restaurant preceded by much loud heaving. That leads all the others in the restaurant to start vomiting too. Even more when several people are pushed/fall into that pool.
Occurs in Guest House Paradiso when the owners of the guest house unknowingly feed their guests contaminated fish. Once one guest starts projectile vomiting into the hall, all the other guests start too, causing an enormous flood of vivid green puke.

Literature

The page quote is from Dave Barry's novel Tricky Business.
In The Body, by Stephen King (which became the movie Stand by Me), a story-within-the-story involves a pie-eating contest, in which one contestant deliberately induced vomiting in himself, and set off a chain reaction among other contestants, officials, and the audience.
Causing one of these is how Buster Casey, aka Rant: An Oral Biography of Buster Casey got his nickname. As a kid he created a haunted house with actual brains, blood and guts from a slaughterhouse. Once the other children realized what they were touching...well, apparently the onomatopeia for hurling is "Raaannntt!"
Happens in Paul Murray's Skippy Dies, after someone spikes the punch bowl at a school dance.

Live-Action TV

One memorable Saturday Night Live skit (from season 20) pushed this gag to the breaking point, having every single character spray puke as the vomit chain reaction spread across the entire country after a cop pukes at the sight of a dead body.
A pregnant Pam of The Office deliberately sets this off in response to her co-workers ignoring her requests for a little consideration for her morning sickness.
Happens about 90% of the time on Hells Kitchen when a losing team is forced to eat disgusting "leftover" parts of animals as punishment for losing a challenge. Inevitably, someone won't be able to keep it down, which will set another couple of people off.
One episode of Dirty Jobs has Mike at the Jelly Belly Jelly Bean factory and one of the staff admits to having vomited while they were making the vomit-flavored jelly beans (most likely from the smell).

Video Games

Vomit chain reactions are one of the biggest obstacles to success in the game Theme Park. One customer hurls, then another sees it and does the same, and pretty soon your entire park is paved in vomit.
- The same happens in Theme Hospital.
There is a memorable quest line in the Burning Crusade expansion of World of Warcraft in which the player is asked to deliver a rotten undead egg to a character named Tobias the Filth Gorger. Upon consuming this vile object, his ensuing belch causes a Vomit Chain Reaction in every NPC (and player) in about fifty yards.
This is very common in Dwarf Fortress, since vomit inspires other dwarves to vomit as well. Because of this and cave adaptation, the front door of your fortress often gets a shallow moat of dwarf puke.

Web Original

The Comedy Machinima film "Male Restroom Etiquette" has a scene where a guy misses and soils the floor, "so his friends 'Ralph' and 'Chuck' begin to... ralph and chuck."

Western Animation

In the Metalocalypse episode "Mordland", this happens and is due to all of the band members being drunk most of the time. Nathan vomits up blood and goes in for another routine liver transplant.
Although they had all taken ipecac for a vomit holding contest, this appears in Family Guy.
- Several other gags involve such a chain reaction spewing; one even dragged it out to an Overly Long Gag. (Those weak of stomach are advised to mute the sound.)
In the short lived sitcom That's My Bush (from the creators of South Park), a Vomit Chain Reaction occurs when one character attempts to get another to vomit by sticking his fingers into his throat and vomiting himself, which inspires others to vomit in turn, leaving everyone in the room vomiting except the person who originally had to.
Happens a couple of times on Total Drama Island.
Happens in one South Park episode, where Cartman and Co write a book which no-one is able to read without vomiting. The book is called "The Adventures Of Scrotie McBoogerballs".
MAD has a Gossip Girl parody named "Gossip Hurl" involving this.