I Don't Find This Particularly Funny -

From Wikipedia:

The Tanganyika Laughter Epidemic of 1962 was an outbreak of mass psychogenic illness (MPI) in the vicinity of the village of Kashasha on the western coast of Lake Victoria in the modern nation of Tanzania near the border of Kenya. Due to its nature the incident has been confused with positive humorous or infectious laughter as seen in phenomena like the holy laughter movement. The nature of MPI, however, is quite dissimilar to these euphoric experiences.

Records of this occurrence, as so often with cases of MPI, are sparse, and have been embellished and misquoted. 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.

Though the epidemic may have started in humor, the oft-noted laughter became significant in an entirely different way. Other more worrisome symptoms were reported on a similarly massive scale; pain, fainting, respiratory problems, rashes, and attacks of crying all appeared to some extent. This laughter epidemic is often misunderstood as implying that thousands of people were continuously laughing for months. As noted above, this is impossible; the true nature of the epidemic was occasional attacks of laughter among groups of people, occurring throughout the noted region at irregular intervals.

No one knows what sparked this incident, but scientists can make reasonable guesses as to why mass hysteria may have affected this part of the world. Independence from Great Britain had been achieved recently, on December 9, 1961, and Kashasha was at the time part of the nation of Tanganyika (Tanganyika would merge with Zanzibar in 1964, creating the modern nation of Tanzania). Students felt that expectations from their teachers and parents had risen markedly, and said they felt stressed as a result. This could explain the epidemic's genesis in a boarding school; one cure for MPI is removing sufferers from their current surroundings, impossible without shutting the school down, something which the administrators were surely reluctant to do. The spread of the epidemic, laughter, crying, rashes, and all, among the adult population may signify widespread uncertainty about the future among Tanganyikans. Situated in the northwestern corner of Tanganyika, the region may have been too isolated and insular to allow for a change of location, which allowed the epidemic to spread and last for a great amount of time. The unique characteristics of the Kashasha area, namely its isolation, a significant population, stress among the entire population and especially the boarding school component, combined perhaps with pure chance, probably best explain why the epidemic occurred and how it lasted so long.