A condition in which a large group of people exhibit the same state of violent mental agitation WordNet 3.0 & Open English WordNet

Mass psychogenic illness, also called mass sociogenic illness, mass psychogenic disorder, epidemic hysteria or mass hysteria, involves the spread of illness symptoms through a population where there is no infectious agent responsible for contagion. Wikipedia

The rapid spread of illness signs and symptoms affecting members of a cohesive group Wikipedia Disambiguation

The sociopsychological phenomenon in which a large group of people exhibit the same or similar hysterical symptoms simultaneously. Wiktionary

Sociopsychological phenomenon. Wiktionary (translation)