I’m talking about mass public retching, as described in fiction such as the movies *Stand by Me *and The Goonies.
Have these ever happened in real life? I know that if there was an illness going around, or a mass exposure to a chemical it’d be possible.
Many years a go I worked one season as a pilot flying air tours in the Grand Canyon. Like this http://www.airvegas.com/tour_info.php?id=24, but in a similar-but-smaller airplane about 2/3rds that size.
My personal record for a single flight with 9 passsengers is 32 bags of puke. All 9 participated; there were no abstentions.
Probably not quite what you meant, but it’s a start.
p.s. Here http://boards.straightdope.com/sdmb/showthread.php?t=451260 is a fine puke-related thread from some years ago with more detail on my aerial puke-a-thon as well as many other poster’s stories of massive but individual gastro-distress.
Go to a local 5K or 10K and watch the finish line. 
Personally, I have a hard time not puking at the sight/sound of someone else puking.
“Hey! Garth, get it together, man. 'Cause if you hurl, and I catch a whiff of it, man… I’m gonna spew. And if I blow chunks, chances are someone else is gonna honk, alright? And that’s gonna set off a parastolic reaction, alright?”
I’ve seen it happen on a plane. Oh, not so dramatic, but even the people that are not feeling badly start to, when they smell all that puke. I was one of the last ones to start throw up.
But it isn’t like people running around and screaming. Especially in a plane where everyone has sickness bags. We all just went for our bags. Probably 2/3s of economy class was sick - I had no idea what was going on in first class.
They were holding on to their monocles and gently vomiting into champagne flutes.
I’ve seen it happen on a boat. Probably close to 400 passengers, of whom about 300 hurled. Amazingly, I was not one of them.
When I got home I showered like you wouldn’t believe.
One of the funniest scenes in any movie, EVER, is the scene in The Sandlot where they all eat the chew and get on the tiltawhirl. I laughed so hard the first 100 times I saw that. Very funny.
At the risk of fancying yet another Just-So Story, I wonder if “contagious puking” was selected for-usually people would all have eaten the same thing (mammoth meat, say), and the first person to get sick might be a warning that tonight’s meal isn’t going to be all that healthy.
I have heard that theory. And I have heard that contageous puking is more common among children, who are typically more sensitive to spoiled or poisonous food.