The Dancing Plague
1518 saw a mass hysteria occur that involved large numbers of people in Strasbourg dancing for days on end without any rest. Known as the Dancing Plague, it began with a single woman dancing in the street before she was slowly joined by more and more people until there were more than 400 taking part. Many of the dancers died from exhaustion or heart attacks.
In 2002, many people in the Indian state of Uttar Pradesh began to suffer from a mass delusion that there were aliens attacking the local area. The telltale signs of an attack were severe scratches and burns on the faces of victims, though nobody was ever seen to exhibit such wounds. The government was forced to intervene after thousands of people became so scared that they wouldn’t go to work or leave their houses.