What caused the dancing plagues (1374, 1518 and others)?

Between the 1200s and the 1500s, crowds along the Rhine and Meuse danced for days, sometimes to collapse. The best recorded outbreaks are Aachen and the Low Countries from 24 June 1374, and Strasbourg from 14 July 1518 (50 to 400 dancers). Two main ideas: ergot poisoning from fungus on rye (which needs a cool, wet June), or a mass stress reaction in people worn down by hunger and disease (Waller). Our test: does the weather before the dances look like the weather before recorded ergot epidemics?