The Dancing Plague outbreak began in July, 1518 when a woman, Frau Troffea, began to dance fervently in a street in Strasbourg, Alsace (then part of the Holy Roman Empire). Her non-stop dance lasted somewhere between four to six days.
Within a week, 34 others had joined, and within a month, there were around 400 dancers. Many took to dancing for days without rest, and — over the period of about one month — some of the people died from heart attack, stroke or exhaustion.
Historical documents, including "physician notes, cathedral sermons, local and regional chronicles, and even notes issued by the Strasbourg city council" are clear that the victims danced. It is not known why these people danced to their deaths.
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 continuously 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.
There is no consensus among modern-day scholars as to the cause of dancing mania. The several theories proposed range from religious cults being behind the processions to people dancing to relieve themselves of stress and put the poverty of the period out of their minds.
It is, however, thought to be as a mass psychogenic illness in which the occurrence of similar physical symptoms, with no known physical cause, affect a large group of people as a form of social influence.
Sources agree that the dancing plague was one of the earliest-recorded forms of mass hysteria, and describe it as a "psychic epidemic."
Whatever happened, the dancing mania appears to have completely died out by the mid-17th century.
Thanks History.com