It has also been called the Dancing Plague of 1518 and "dancing mania".
Was it a social disease of mass hysteria?
Was it a neurological malfunction?
Was it caused by a blood disorder?
Was it caused by poisoned food (ergot) ?
Was it caused by a tarantula bite?
Was it actually demon possession?
The effects of the Black Death had not yet subsided, and the graves of millions of its victims were scarcely closed, when a strange delusion arose in Germany, which took possession of the minds of men, and, in spite of the divinity of our nature, hurried away body and soul into the magic circle of hellish superstition. It was a convulsion which in the most extraordinary manner infuriated the human frame, and excited the astonishment of contemporaries for more than two centuries, since which time it has never reappeared.
While dancing they neither saw nor heard, being insensible to external impressions through the senses, but were haunted by visions, their fancies conjuring up spirits whose names they shrieked out; and some of them afterwards asserted that they felt as if they had been immersed in a stream of blood, which obliged them to leap so high. Others, during the paroxysm, saw the heavens open and the Saviour enthroned with the Virgin Mary, according as the religious notions of the age were strangely and variously reflected in their imaginations.
http://biotech.law.lsu.edu/Books/hecker/Death12.htm
http://biotech.law.lsu.edu/Books/hecker/Death_c.htm
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.
Historical documents, including "physician notes, cathedral sermons, local and regional chronicles, and even notes issued by the Strasbourg city council" are clear that the victims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. [1] It is not known why these people danced to their deaths, nor is it clear that they were dancing willfully.
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".
...it involved groups of people, sometimes thousands at a time, who danced uncontrollably and bizarrely, seemingly possessed by the devil.
One of the first major outbreaks was in Aachen, Germany, on June 24 , 1374 ; the populace danced wildly through the streets, screaming of visions and hallucinations, and even continued to writhe and twist after they were too exhausted to stand.
Having occurred to thousands of people across several centuries, dancing mania was not a local event, and was, therefore, well-documented in contemporary writings. More outbreaks were reported in the Netherlands, Cologne, Metz, and later Strasbourg (Dancing Plague of 1518), apparently following pilgrimage routes. [3]
http://en.wikipedia.org/wiki/Dancing_Plague_of_1518
(St. John's Dance)......phenomenon which emerged during the time of the Black Death. The medical term is chorea imagnativa aestimative. Basically, it is a form of apraxia expressing itself as "dancing rage," as uncontrolled ecstatic body movements. In the eyes of the church, those suffering from St. John's Dance were possessed by the devil.
http://en.wikipedia.org/wiki/Dancing_mania
The learned Nicholas Perotti gives the earliest account of this strange disorder. Nobody had the least doubt that it was caused by the bite of the tarantula, a ground-spider common in Apulia: and the fear of this insect was so general that its bite was in all probability much oftener imagined, or the sting of some other kind of insect mistaken for it, than actually received. The word tarantula is apparently the same as terrantola, a name given by the Italians to the stellio of the old Romans, which was a kind of lizard, said to be poisonous, and invested by credulity with such extraordinary qualities, that, like the serpent of the Mosaic account of the Creation, it personified, in the imaginations of the vulgar, the notion of cunning, so that even the jurists designated a cunning fraud by the appellation of a "stellionatus." Perotti expressly assures us that this reptile was called by the Romans tarantula; and since he himself, who was one of the most distinguished authors of his time, strangely confounds spiders and lizards together, so that he considers the Apulian tarantula, which he ranks among the class of spiders, to have the same meaning as the kind of lizard called [Greek text], it is the less extraordinary that the unlearned country people of Apulia should confound the much-dreaded ground-spider with the fabulous star- lizard, and appropriate to the one the name of the other.
http://biotech.law.lsu.edu/Books/hecker/Death20.htm
There have been suggestions of the cause being: demonic possesion, ergot poisoning, murine typhus epidemic, epilepsy, and spider bite. However, some historians believe the psychological impact of a barrage of natural calamities caused a neurological change and premise that hardships endured drove them into a frenzied state of mind and body in epic proportion.