The Dancing Plague (or Dance Epidemic) of 1518 was a case of dancing mania that occurred in Strasbourg, Alsace (then part of the Holy Roman Empire) in July 1518. Around 400 people took to dancing for days without rest, and, over the period of about one month, some of those affected died of heart attack, stroke, or exhaustion.
In July, 1518, a woman referred to as Frau Troffea stepped into a narrow street in Strasbourg, France and began a fervent dancing vigil that lasted between four and six days. By the end of the week, 34 others had joined her and, within a month, the crowd of dancing, hopping and leaping individuals had swelled to 400.
Source: http://www.digitaljournal.com/article/258521, https://en.wikipedia.org/wiki/Dancing_Plague_of_1518