Tanganyika — now called Tanzania — is one of the oldest known inhabited places on the planet, with evidence of humans and pre-humans dating back millions of years. With a history of that length, it is almost guaranteed that something incredibly odd would have happened there. On January 30, 1962, that promise was fulfilled. That day, three schoolgirls at a small, mission-run boarding school in the village of Kashasha started laughing.

That would be nothing special in and of itself, of course — it would be very difficult to find a school where a single day goes by without a group of three or so girls (or boys, for that matter) laughing at something. And what happened next in Tanganyika is, similarly, a non-story: more children laughed.

And more, and more, and more. At one point, 95 of the 159 students in the school were caught up in the laughing craze — with no explanation. The laughter was not prompted by a joke or silly event, or, really, anything. It just seemed to spread haphazardly much like a virus. And it was not a simple “hah” here or there. Those struck with the giggles laughed for hours on end, and some were laughing for days. In total, according to researcher Christian F. Hempelmann, Kashasha and surrounding villages were overcome with uncontrollable laughter intermittently for over a year, and perhaps as long as 18 months. Over a dozen schools temporarily closed because students were too distracted to learn, and about 1,000 people were laughing, uncontrollable, and seemingly without any cause for jest.

Hempelmann believes that the Tanganykian children were under a lot of stress — more so than most typical students — and the laughter epidemic was a byproduct of this and a lack of another outlet for that stress. When a few girls started laughing, it opened the door for others to do so as well, to an unimaginable extreme. But that’s mostly a guess. The cause is a mystery — there is no environmental or medically detectable explanation known, even today. The best explanation is mass hysteria or “mass psychogenic illness,” a not well understood phenomenon in which is, unfortunately, described by its symptoms and not the underlying cause.

What caused the epidemic to abate? That, too, is a mystery. As suddenly and inexplicably as the laughing contagion began, it ended.

Bonus fact : According to IMDb, Robin Williams, when interviewed on an episode of Inside the Actors Studio, caused an audience member to laugh so hard that an ambulance had to be called. The audience member laughed himself into a hernia.

From the Archives: (1) Laughing to Death: When laughing is a symptom of a fatal disease. (2) The World’s Shortest War: Another part of modern-day Tanzania’s history.

Related: Robin Williams’ Inside the Actors Studio performance. Watch at your own risk.