" " Girls sit in a classroom in Namibia, 1969. Back in 1962 in Tanzania (then Tanganyika), a laughing epidemic broke out at a girls' school. Jochen Blume/ullstein bild via Getty Images

At a small girls' boarding school in Tanganyika (now Tanzania), three students started to giggle. Starting and stopping abruptly, their fits would last anywhere from a minute or two to several hours. This "laughter" proved contagious — soon other girls were doing the same thing. No one could concentrate on their schoolwork, and restraining the laughing students proved ineffective. Six weeks later, more than half of the school's middle and high schoolers had caught the laughing bug.

School officials shut the place down. But when they reopened it two months later, the laughing plague immediately restarted and the school was once again shuttered. The laughing epidemic spread to other schools and lasted somewhere between six and 18 months [sources: McGraw and Warner, Sebastian].

So what caused this? "The bad news is, it had nothing to do with humor. There was no merriment. Laughter was one of many symptoms," said linguist Christian F. Hempelmann, who researched the incident. He noted that the students also had fits of pain, fainting, crying and rashes.

He blamed excessive stress for the uncontrollable giggles. The boarding school where the laughter began was a very strict one. Plus the country had just gained its independence, and people were anxious about the future. With all of the terrorism in the world today, experts say another laughing epidemic wouldn't be surprising [sources: McGraw and Warner, Sebastian].