Mass hysteria

Dancing plagues of the Middle Ages are thought to have been caused by mass hysteria

Mass hysteria or mass psychological illness is a phenomenon when a large group of people share a common emotion. This emotion can be positive, such as people being euphoric at a rock concert, or it can be negative, like after a great person or statesman died. It was first used by the Quarteerly Christian Spectator in 1830, to describe an outbreak of cholera.