It was centuries ago, right around this time, when the city of Aachen, Germany, was struck by an unusual outbreak that, legend says, spread as far away as Madagascar.
According to the scant written accounts there are of the outbreak known as St. Vitus' Dance, back in 1374, groups of people -- sometimes thousands at a time -- started dancing uncontrollably. It continued for days, and in some cases, weeks and months. Some people reportedly danced until they collapsed from exhaustion or even death, while others suffered heart attacks and broken bones.
"One written account described people as united by one common delusion," said James Fowler, professor of medical genetics and political science at the University of California, San Diego.
The dancers also seemed to hallucinate and lose control of their senses, Fowler said.
To this day, no one knows what led so many people into the dancing fits. At the time, many people believed it was a curse.
Fowler and other experts, however, believe the 14th-century dancing outbreak was an early example of social contagion. Just as yawning and laughing seem to be contagious, experts say manic dancing can be as well.
St. Vitus' Dance, which began exactly 637 years ago last Friday, is also a medical condition known as Sydenham chorea, named after the doctor who discovered it. The disease seemed to resemble the manic dancing that took place in Europe in honor of St. Vitus, the patron saint of entertainers.
The National Institute of Neurological Disorders and Stroke describes Sydenham chorea as a central nervous system disorder characterized by jerking movements mostly of the face, arms, legs and hands. It's caused by infection by the same bacteria that cause rheumatic fever.
But as far as the early dancing outbreak is concerned, it's consistent with the belief that people tend to mimic others around them.
"We tend to imitate body language and the emotional state of people that we're with, and we're not aware of it," Fowler said.
John Cacioppo, director of the University of Chicago's Center for Cognitive and Social Neuroscience, said, "A couple of different processes contribute to something like this. One is peer pressure. If you're sittng in a setting and everyone is acting in a certain way, you do the same thing."
There's also a concept called pluralistic ignorance, which Cacioppo described as, "I would stop, but I don't see anyone else stopping."
In his book "Connected," California's Fowler described St. Vitus' Dance in some detail and also talked about other episodes of mass hysteria, or mass psychogenic illness, including a 1962 outbreak of laughter among schoolgirls in Tanzania.
"Someone has an intense emotional experience, and because of its intensity, passes it on to other people," Fowler explained.
Past research has suggested that mirror neurons contribute to this behavior.
"These are neurons in the part of the brain that you use to move your own body," Fowler said. "They fire when someone else is doing something."
Mirror neurons are at work often, though they only rarely cause episodes such as St. Vitus' Dance or the African laughing outbreak.
"It's unusual to find it in such a concentrated form, but it's easy to find contagion and mimicry every day," Cacioppo says. "Even babies show it."