Sudden bouts of contagious dancing plagued pre-modern Europe – afflicting up to a thousand people at a time.

In July 1518, a woman (who may have been named “Frau Troffea”) stepped into a street in Strasbourg and began to dance. A few other women joined in, and then more and more people. Numbers from contemporary accounts vary, of course, but up to four hundred residents of the city were caught up in this uncontrollable dance.
This was not a unique occurrence. Dancing plagues are recorded many times throughout Medieval and early modern Europe: in Augsburg, Zurich, and more than once in Strasbourg. A 1374 bout from Aachen in Germany supposedly spread to Belgium and the Netherlands before it stopped. A dancing plague in 1237 afflicted many German children (and may have been one of the origins of the Pied Piper of Hamlin story). The descriptions were consistent – people caught up in an irrepressible desire to move their arms and legs in a twitching, dance-like fashion – but the cause remains unknown.
There are theories, of course, both contemporary and modern: mass hysteria, a kind of group hypnotic state brought about by times of extreme hardship; food poisoning from fungus-infected grain; possession by demons; religious cults using the “plague” as an excuse to hide their revelry; and the bite of the wolf spider. That last one was connected to a similar condition seen in southern Italy called tarantism (after the tarantula wolf spider) – the tarantella dance had its origins as a treatment for this dance mania.
Most people danced themselves to exhaustion. Some apparently danced themselves to death. The dancing could last for hours, days, or weeks; most of the time it just stopped on its own. In some cases people tried to treat the dancing with music, but that just seems wrong to me. You see people involuntarily dancing, so you call in the musicians to accompany them? Sheesh, Medieval medicine was wild. The German physician Paracelsus (the same one who invented elementals) was convinced that the problem was physical and not spiritual, and although he proposed a number of treatments there’s no record of any of them working. Whatever the cause and despite the lack of a cure, this curious phenomenon had more or less disappeared by the end of the 17th century.