An Epidemic of Laughing in the Bukoba District of Tanganyika in 1964

from wikipedia:

The epidemic seems to have started within a small group of students in a boarding school, possibly triggered by a joke. Laughter, as is commonly known, is in some sense contagious, and for whatever reason in this case the laughter perpetuated itself, far transcending its original cause. Since it is physiologically impossible to laugh for much more than a few minutes at a time, the laughter must have made itself known sporadically, though reportedly it was incapacitating when it struck. The school from which the epidemic sprang was shut down; the children and parents transmitted it to the surrounding area. Other schools, Kashasha itself, and another village, comprising thousands of people, were all affected to some degree. Six to eighteen months after it started, the phenomenon died off.

I first hear about it on radio lab , an NPR science podcast.
It is real! You can Google it for medical journal stuff – I found some stuff but it was like reading in Latin!

this link was a bit more understandable
read more if you dare!

this link is for the American scientist.org website article