"While dancing they neither saw nor heard, being insensible to external impressions [with the exception, one might guess, of the music they danced to] .... but were haunted by visions, their fancies conjuring up spirits whose names they shrieked out... Others, during the paroxysm, saw the heavens open and the Saviour enthroned with the Virgin Mary."(14)
"Hence the Church authorities' worry that the "manias" represented a new form or heresy: Nothing is more threatening to a hierarchical religion than the possibility of ordinary laypeople finding their own way into the presence of the gods."
See also Wikipedia. I particularly like the descriptions of Strasbourg's Dancing Plague of 1518, where local authorities hired musicians to play alongside the dancers and keep them dancing until the dance was danced out of them. The less fun parts are the reports of dancers dancing themselves to exhaustion and death.
In a similar vein, there are some reports of epidemics of laughter.