Aug. 1, 2008 -- In July of 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.

Authorities prescribed "more dancing" to cure the tormented movers but, by summer's end, dozens in the Alsatian city had died of heart attacks, strokes and sheer exhaustion due to nonstop dancing.

For centuries this bizarre event, known variously as the dancing plague or epidemic of 1518, has stumped scientists attempting to find a cause for the mindless, intense and ultimately deadly dance. Historian John Waller, author of the forthcoming book, "A Time to Dance, A Time to Die: The Extraordinary Story of the Dancing Plague of 1518," studied the illness at length and has solved the mystery.

"That the event took place is undisputed," said Waller, a Michigan State University professor who has also authored a paper on the topic, which has been accepted for publication in the journal Endeavour.

Waller explained that historical records documenting the dancing deaths, such as physician notes, cathedral sermons, local and regional chronicles, and even notes issued by the Strasbourg city council during the height of the boogying rage, all "are unambiguous on the fact that (victims) danced."

"These people were not just trembling, shaking or convulsing; although they were entranced, their arms and legs were moving as if they were purposefully dancing," he said.

Possible Causes

Eugene Backman, author of the 1952 book "Religious Dances in the Christian Church and in Popular Medicine," sought a biological or chemical origin for the dancing mania. Backman and other experts at the time believed the most likely explanation was ergot, a mold that grows on the stalks of damp rye. When consumed unknowingly in bread, the mold can trigger violent convulsion and delusions but not, Waller says, "coordinated movements that last for days."

While at Australia's James Cook University, sociologist Robert Bartholomew proposed a theory that the dancers were performing an ecstatic ritual of a heretical sect, but Waller counters, "there is no evidence that the dancers wanted to dance."

"On the contrary," he added, "they expressed fear and desperation," according to the written accounts.

Unusual Events Preceded the Epidemic

A series of famines, resulting from bitter cold winters, scorching summers, sudden crop frosts and terrifying hailstorms, preceded the maniacal dancing, Waller said. Waves of deaths followed from malnutrition. People who survived were often forced to slaughter all of their farm animals, secure loans and finally, take to the streets begging.

Smallpox, syphilis, leprosy and even a new disease known as "the English sweat" swept through the area.

"Anxiety and false fears gripped the region," Waller said.

One of these fears, originating from a Christian church legend, was that if anyone provoked the wrath of Saint Vitus, a Sicilian martyred in 303 A.D., he would send down plagues of compulsive dancing.

Waller therefore believes a phenomenon known as "mass psychogenic illness," a form of mass hysteria usually preceded by intolerable levels of psychological distress, caused the dancing epidemic.