It's an open question. There are, in fact, rumors of such things happening in the world. They are little more than that, but like many urban legends, what's most interesting about them is that they could be true. Take, for instance, the so-called "Tanganyika Laughter Epidemic," which (supposedly) occurred in a small village in Tanzania in 1962. Here's the legend (thanks, 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.
But is such a thing even possible? On a much smaller scale, it's a phenomenon all of us have witnessed; with a little extrapolation, it's not tough to imagine a 6-month bout of infectious laughter. In case any of you don't know what we're talking about, here's a great example from YouTube, of an otherwise lame comedy routine made great by infectious, incapacitating laughter: