One of the unfortunate things about having a skeptical approach is that sometimes, you have to admit you simply don't have an explanation.
I get that it's frustrating. I used to run into this sometimes with students, and have conversations like the following:
Student: Do you think there's intelligent life elsewhere in the galaxy?
Me: I don't know.
Student: But what do you think?
Me: I don't think anything. I simply don't know. We have one example of a planet with intelligent life, and only vague guesses about how likely the conditions are that would select for intelligence. It might be extremely common, or it might be extraordinarily rare. We just don't know.
Student: Doesn't that drive you crazy?
Yes, sometimes it does drive me crazy. But if you're approaching the world scientifically, you better get used to it, because you're going to be spending a lot of time standing right up against what astrophysicist Neil deGrasse Tyson calls "the perimeter of our own ignorance."
And you don't have to go to the outer reaches of the galaxy to find phenomena that we've yet to explain -- ones for which, if you consider them honestly, you have to admit you may never have a good explanation. The universe is big and weird and chaotic and complex, and frankly, we're lucky we've been able to explain as much of it as we have.
Which brings us to the Dancing Plague of 1518.
In July of the year 1518, in the town of Strasbourg, Alsace, a woman known to us only as Frau Troffea suddenly felt compelled to dance. Unable to stop herself, she left her house and began to dance on the street, resisting all attempts to get her to stop.
Keep in mind that this was a highly superstitious time, when such behavior wouldn't have been considered comical; the early sixteenth century was the era of witch burnings and the heresy-hunters of the Inquisition. To the onlookers of the time, Frau Troffea didn't seem funny, she looked as if she'd been possessed by a demon.
Worse, several other people joined her over the hours that followed. During the next week, three dozen people were dancing; by mid-August, the numbers had risen to four hundred, and the illness -- whatever it was -- had spread to nearby towns. At first, both the doctors and religious authorities suggested the victims be encouraged to dance themselves to exhaustion, to "dance free of it," and even hired musicians to keep them going. But as the "dancing plague" spread through the countryside, panic ensued. The powers-that-be reversed course, and forbade musicians from egging the dancers on. The priests and bishops declared that the dancers were being punished by Saint Vitus, the patron saint of dancing, but what they'd done to merit that was never clear, as the dancers came from all walks of life. Despite that, and probably driven by a desperation to do something, the religious authorities forced the dancers to wear shoes blessed with Holy Water, which had crosses embroidered on them, in the hopes that this might make the saint happy and stop the strange affliction.
Unsurprisingly, this had no effect whatsoever.
By September, the whole thing began to die down. Some contemporaneous sources say a few of the dancers danced themselves to death, but the number of fatalities (if any) are uncertain. In the third week of September, the afflicted (now over their bad case of Boogie Fever) were sent to the shrine of Saint Vitus to receive absolution, and the whole episode ended.
So, what caused this bizarre outbreak?
If you're discounting the Demonic Possession Hypothesis and the Pissing Off Saint Vitus Hypothesis, there are two explanations that are most commonly proffered to account for the Dancing Plague, but both of them are not without their problems.
The first is that it was ergotism -- a condition caused by eating ergot-infected wheat and rye. Ergot is a fungus that produces a chemical analog to LSD, and when consumed, it can cause bizarre hallucinations. While this is a possibility, there are two main arguments against it. First, an LSD trip doesn't last for weeks, and some of the people affected danced through most of July and August. Second, severe ergotism -- consumption of large quantities of the fungus-infected grain -- triggers another effect of the chemical, which is vasoconstriction. People with severe ergotism can have blood vessel constriction bad enough to cause gangrene in their extremities. Considering how long the Dancing Plague went on, it's odd that if it was ergot, no one showed the other symptoms that usually come along with it.
The second is that it was an example of mass psychogenic illness. This occurs when groups of people start exhibiting similar symptoms because of being part of a cohesive group and sharing similar biases and living conditions. Put simply, it was superstition, hysteria, and the power of suggestion at work. Examples of other illness thought to be caused by this phenomenon are the Tanganyika Laughter Epidemic of Tanzania and the "June Bug" incident in the southern United States, both of which (coincidentally) happened in 1962. More controversially, some have explained Havana syndrome and Morgellons disease as psychogenic in origin -- but there are plenty of people who dispute both of those.
But as far as the Dancing Plague goes, there is one odd fact that argues against it being psychogenic in origin. Almost every victim of the outbreak lived near water -- particularly along the Rhine and Moselle Rivers. The farther away you were from the rivers, the less likely you were to be affected. This gives the appearance of some sort of water-borne disease, but there's no known germ that has these effects.
Whatever caused the Dancing Plague -- and we still don't have an explanation that accounts for all of the known facts -- at least the authorities of the time didn't do what you might expect, which is to turn against the victims. Considering the medieval tendency to see Satan hiding in every dark corner, it's kind of surprising they didn't. There's no indication that, even after having spent a few weeks gettin' down, the victims were treated any differently afterward.
Maybe it was the trip to Saint Vitus's shrine that did the trick.
In any case, we really don't know what caused it. Frustrating, but -- to come back around to my initial point -- given how weird and complicated the world is, that's gonna happen. And as good skeptics, we have to be okay with it. We can't explain everything, and even given all the facts at hand, there will still be times we have to shrug our shoulders and admit we don't know.
Even if it does drive us crazy.