An interesting report by Discovery's Jennifer Viegas on the "Dancing Plague" of 1518 (often attributed to ergotism) discusses the possible psychological roots of this mass behavioral phenomenon. Also mentioned was a more modern variety of psychogenic disorder -- an epidemic of hysterical laughter originating in a Tanzanian boarding school:
Perhaps the most unusual documented case of mass psychogenic illness was the Tanganyika Laughter Epidemic of 1962. A paper published the following year in the Central African Journal of Medicine described what happened.
Triggered by a joke among students at a Tanzania boarding school, young girls began to laugh uncontrollably. At first there were spurts of laughter, which extended to hours and then days.
The victims, virtually all female, suffered pain, fainting, respiratory problems, rashes and crying attacks, all related to the hysterical laughter. Proving the old adage that laughter can be contagious, the epidemic spread to the parents of the students as well as to other schools and surrounding villages.
Eighteen months passed before the laughter epidemic ended.