Watching someone retch can induce feelings of sickness in the watchers. Retching is more contagious than yawning and laughter - except when these latter two reactions are undertaken simultaneously whilst eating a biscuit. In which case, retching can also occur.
Watching someone else retch is likely to cause two thirds of people to have a good retch themselves. Such widespread co-vomiting is dangerous and many inner-city streets are unable to cope with it.
"It's sickening how this suddenly happens - even in 75 per cent of ordinary people who have just spent a pleasant evening enjoying an Indian take-away," said one leading vomitologist, before throwing up some more statistics.
Now scientists think they have solved the mystery of why retching is so contagious. It is, they say, all down to the way some people make each other sick.
"When people say to you: 'You make me sick!'," says Dr Eva Lott, "it's usually true. And then watching them being sick makes us sick."
After 20 years of research into retching, Dr Lott is retiring from the Retching Research Unit at the University of Throweenup. "I'm sick of it," she said.