Dancing is typically associated with happiness and celebration. If you see someone dancing, you are safe to assume that person is having a good time. You would have been wrong in your assumption, however, if you happened to witness the spectacle that happened 500 years ago when an entire town started dancing — and kept dancing — for no apparent reason.

The phenomenon known as the Dancing Plague of 1518 began in July of that year when a woman by the name of Mrs. Troffea was witnessed dancing in the streets of Strasbourg, modern-day France. Her facial expression did not match up with the apparent joyfulness of her body’s movements. She looked dazed, troubled, and fearful.

Mrs. Troffea’s dance continued, uninterrupted, for four to six days. By that time 34 others joined in. They, like Mrs. Troffea, were unable to control themselves. To say they couldn’t control themselves does not mean they found the dance music so captivating that they just had to dance; they were physically unable to stop the energetic dancing motion of their bodies.

Within a month, more than 400 residents of Strasbourg were on the streets in the non-stop, out-of-control dance-off to the death. The unrelenting exercise proved to be too much for some of the victims. Many passed out from exhaustion, while others died from heart failure or strokes. According to one report, as many as fifteen people per day expired from the Dancing Plague. John Waller, a historian who studied the Dancing Plague of 1518, opined that a marathon runner would be unlikely to withstand the intense cardio exercise of the victims of the plague.

It may sound like something out of badly-written science fiction, but the contemporary records well document the event. Physician records, clergy sermons, and city council records all confirm the mysterious phenomenon that compelled so many people to dance to their death.

As the phenomenon continued and afflicted more and more people, local authorities looked to a number of explanations. After much debate, they determined that the most promising cure for the dancing plague was to encourage more dancing. They opened large public arenas and even built a large wooden stage and brought in musicians to provide some music to go with the dancing. This approach, alas, was less than effective. It resulted in an epidemic-like spread of the affliction.

After a month, just as mysteriously as it began, the Dancing Plague ended. Dazed and exhausted Strasbourg residents returned to their dance-free lives, never understanding what caused the town to lose itself to the unheard music. After 500 years, we still aren’t sure why the event happened.

Today, a number of theories exist. Many researchers believe the residents of Strasbourg suffered the bizarre side effects of food-poisoning from ergot fungi. This hallucinogenic substance commonly on grains in the wheat family and can be found in rye bread. Ergotamine, the principal psychoactive product of ergot fungi, is structurally related to the drug lysergic acid diethylamide (LSD-25), and is the substance from which LSD-25 was originally synthesized. Researchers have linked the same substance to other mass outbreaks of seemingly-unexplainable behavior, including the Salem Witch Trials.

Others are not so quick to chalk the Dancing Plague up to a case of fungus-induced LSD poisoning. One of the problems with the theory is that ingestion of ergot fungus commonly causes gangrene, due to restriction of blood flow to the extremities. It is unlikely that those suffering from such a large intake of the toxic substance would have been able to stay on their feet, much less be able to dance for days on end. Additionally, critics argue that it is unlikely that so many people would have been affected in identical ways by ingestion of psychotropic chemicals.

Another theory suggests that the residents of Strasbourg suffered from “stress-induced psychosis,” triggered by malnutrition, sickness, and generally-poor living conditions.

Seven other outbreaks of Dancing Plague have been recorded during the Middle Ages, (assuming, of course, that the Middle Ages actually existed and wasn’t a hoax), although none were as widespread or as well documented as the Dancing Plague of 1518. Whether it was caused by bread fungus, mass hysteria, or some other cause, the next time you feel an uncontrollable urge to tap your feet or break out some fancy moves, you may want to double check that there is actually a good reason for you to do so.