hmm

In 1374, hundreds of people along the River Rhine compulsively danced for days at a time, swept up in a terrifying mania of mass, compulsive, dancing. The hysteria spread through north-Eastern France and the Netherlands, lasting for months. Similar "dancing epidemics" broke out over the next two centuries. The new issue of The Psychologist features a scientific look at this incredibly strange kind of hysteria. From The Psychologist:

An important clue to the cause of these bizarre outbreaks lies in the fact that
they appear to have involved dissociative trance, a condition involving (among
other things) a dramatic loss of self-control. It is hard to imagine people
dancing for several days, with bruised and bloodied feet, except in an altered
state of consciousness. But we also have eyewitness evidence that they were not
fully conscious. Onlookers spoke of the dancing maniacs of 1374 as wild,
frenzied and seeing visions. One noted that while ‘they danced their minds were
no longer clear’ and another spoke of how, having wearied themselves through
dancing and jumping, they went ‘raging like beasts over the land’ (Backman,
1952). The hundreds of possessed nuns described in chronicles, legal records,
theological texts or the archives of the Catholic Inquisition were equally
subject to dissociative trance (Newman, 1998; Rosen, 1968). Some may have
simulated the behaviour of the demoniac as a means of eliciting positive
attention (Walker, 1981), but the detailed descriptions of astute and cautious
inquisitors leave little doubt that most were genuinely entranced.