From Wikipedia's article: "The Dancing Plague (or Dance Epidemic) of 1518 was a case of dancing mania that occurred in Strasbourg, France (then part of the Holy Roman Empire) in July 1518. Numerous people took to dancing for days without rest, and, over the period of about one month, most of the people died from heart attack, stroke, or exhaustion".
An important thing to note here is the fact that this is just one case of Dancing Mania... which means that this isn't an isolated case. So there's not just one but several cases of large groups of people spontaneously dancing for days on end, for no apparent reason, until they died from exhaustion!
"The outbreak began in July 1518, when a woman, Frau Troffea, began to dance fervently in a street in Strasbourg. This lasted somewhere between four to six days. Within a week, 34 others had joined, and within a month, there were around 400 dancers. Many of these people eventually died from heart attack, stroke, or exhaustion".
That Frau Toffea... she really knew how to throw a party didn't she? Now remember, this is 1518. It's not like these people were hopped up on Meth or Ecstasy. Frau wasn't bouncing around with a doctor's mask on, waving around glow sticks. The article goes on to say that ergot poisoning, whose symptoms include delirium and hallucinations could have been the cause, but it's highly unlikely as another symptom of ergotism is a loss of blood to the limbs which makes coordinated movement difficult. So what was the cause? Nobody really knows. Their best guess is Mass Psychogenic Illness / Mass Hysteria:
"Historian John Waller thinks that the dancing epidemic was caused by mass psychogenic illness (MPI), a manifestation of mass hysteria that is often preceded by extreme levels of psychological distress. Waller states that famine had been prevalent in the region for some time, caused by very cold winters, very hot summers, crop frosts, and violent hailstorms. Mass deaths followed from malnutrition, and those who survived were forced to kill their farm animals, take out loans, and perhaps even beg in the streets. In addition to food shortages, diseases such as smallpox, syphilis, leprosy, and "the English sweat" (a new disease) afflicted the populace, as well as "spiritual despair on a scale unknown for generations." This series of events might have triggered the MPI".
So, you're the Mayor of crazy-town and all of your subjects can't stop dancing. What do you do? According to the article, you say fuck it and throw a party! That's right, these people are flailing around and dancing uncontrollably to their freaking deaths and the powers-that-be come up with the bright idea of building a stage and hiring a fucking band!
"As the dancing plague worsened, concerned nobles sought the advice of local physicians, who ruled out astrological and supernatural causes, instead announcing that the plague was a "natural disease" caused by "hot blood". However, instead of prescribing bleeding, authorities encouraged more dancing, in part by opening two guildhalls and a grain market, and even constructing a wooden stage. The authorities did this because they believed that the dancers would only recover if they danced continually night and day. To increase the effectiveness of the cure, authorities even paid for musicians to keep the afflicted moving. Some of the dancers were taken to a shrine, where they sought a cure for their affliction".
I get that this was before modern psychology and medications and stuff but, come on. They couldn't think of any other options? Note that the article said that most of them died, not 1 or 2 or some, MOST. It took around 400 people dropping dead before they decided that maaaaybe they were going about things the wrong way. Or, maybe they were the 15th century version of candy-ravers and they did know what they were doing.
