Why do humans laugh? (Hint: It’s rarely because something’s funny.)

On Jan. 30, 1962, three schoolgirls started giggling in a boarding school classroom in the northeastern corner of what is now Tanzania—and touched off a very strange epidemic. The three couldn’t stop laughing—and soon the uncontrollable cackles spread to their classmates. The laughing attacks lasted from a few minutes up to a few hours; one poor girl reportedly experienced symptoms for 16 straight days. Victims couldn’t focus on their schoolwork, and would lash out if others tried to restrain them.

When 95 of the school’s 159 pupils had come down with what came to be known as omuneepo, the Swahili word for laughing disease, the school shut down. The students returned to their villages, taking omuneepo with them. The affliction spread from person to person, school to school, village to village. “The education of the children is being seriously interfered with and there is considerable fear among the village communities,” noted local medical officers in a 1963 report in the Central African Journal of Medicine. They could find no explanation for the matter. When the epidemic finally died down months later, roughly a thousand people had been struck by the “laughing disease.”

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Laughter is a vexing subject even when it’s not spreading through the countryside like a virulent disease. Take the work of Robert Provine, a neuroscientist and psychology professor at the University of Maryland, Baltimore County. For his book, Laughter: A Scientific Investigation, Provine engaged in what he called “sidewalk neuroscience,” tracking and observing real-world laughter. He and his collaborators used tape recorders to capture more than a thousand “laugh episodes” in bars, shopping malls, cocktail parties, and class reunions. And he had dozens of student volunteers note in a “laugh log” the circumstances around every time they tittered, chuckled, or guffawed.