Six-hundred and forty two years ago today, citizens in the German city of Aachen started to pour out of their houses and into the streets where they began to writhe and whirl uncontrollably. This was the first major outbreak of dancing plague or choreomania and it would spread across Europe in the next several years.

To this day, experts aren't sure what caused the frenzy, which could drive those who danced to exhaustion. The outbreak in Germany was called St. John's dance, but it wasn't the first appearance of the mania or the last, according to The Black Death and The Dancing Mania, originally published in 1888. In the book, Justus Friedrich Karl Hecker imaginatively describes the spectacle of St. John's dance as follows:

They formed circles hand in hand, and appearing to have lost all control over their senses, continued dancing, regardless of the bystanders, for hours together, in wild delirium, until at length they fell to the ground in a state of exhaustion. They then complained of extreme oppression, and groaned as if in the agonies of death, until they were swathed in cloths bound tightly round their waists, upon which they again recovered, and remained free from complaint until the next attack.

The "disease" spread to Liege, Utrecht, Tongres and other towns in the Netherlands and Belgium, up and down the Rhine river. In other times and other forms the mania started to be called St. Vitus' dance. During the Middle Ages, the church held that the dancers had been possessed by the devil or perhaps cursed by a saint. Called Tarantism in Italy, it was believed the dancing was either brought on by the bite of a spider or a way to work out the poisons the arachnid had injected.

More modern interpretations have blamed a toxin produced by fungus that grew on rye. Ergot poisoning, or ergotism, could bring on hallucinations, spasms and delusions thanks to the psychoactive chemicals produced by the fungus Claviceps purpurea, writes Steven Gilbert for the Toxipedia.

But not all of the regions affected by the strange compulsion to dance would been home to people who consumed rye, points out Robert E. Bartholomew in an article for the July/August 2000 issue of Skeptical Inquirer. Furthermore, the outbreaks didn't always happen during the wet season when the fungus would have grown.

St. Vitus' dance later came to mean Sydenham chorea, a disorder that struck children and did cause involuntary tremors in the arms, legs and face. However those twitches were not the kind of dancing described in the outbreaks of dancing mania.

Another notable epidemic broke out in the city of Strasbourg in 1518. It started in July when a woman called Frau Troffea began to dance. Within a month, 400 people joined in the madness. This plague in particular was probably worsened by apparently well-meaning officials who thought that the victims just needed to dance it out and shake it off. They set aside guild halls for the dancers, hired professional pipe and drum players and dancers to keep people inspired, writes John Waller for BBC.com.

Madness is ultimately what some experts think caused such a bizarre phenomenon. Waller explains that in 1518, the people of Strasbourg were struggling to deal with famine, disease and the belief that supernatural forces could force them to dance. In 1374, the region near the Rhine was suffering from the aftermath of another, true plague: the Black Death. Waller argues that the dancers were under extreme psychological distress and were able to enter a trance state—something they would need to dance for such a long period of time. He blames the dancing mania on a kind of mass hysteria.

Bartholomew disagrees. He points out that records from the time claim that the dancers were often from other regions. They were religious pilgrims, he posits. He writes:

The behavior of these dancers was described as strange, because while exhibiting actions that were part of the Christian tradition, and paying homage to Jesus, Mary, and various saints at chapels and shrines, other elements were foreign. Radulphus de Rivo’s chronicle Decani Tongrensis states that “in their songs they uttered the names of devils never before heard of . . . this strange sect.” Petrus de Herenthal writes in Vita Gregorii XI: “There came to Aachen . . . a curious sect.” The Chronicon Belgicum Magnumdescribes the participants as “a sect of dancers.”

Once the first dancers started their strange ritual, other people perhaps joined in, claiming to be overwhelmed by a compulsion. Societal prohibitions against such unrestrained behavior could then be cast aside.

Ultimately, the cause of choreomania seems to be mystery, but it will never cease to be a fascinating part of European history.