mass hysteria

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. A condition in which a large group of people exhibit similar physical or emotional symptoms, such as anxiety or extreme excitement. Also called epidemic hysteria.

from Wiktionary, Creative Commons Attribution/Share-Alike License

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• n. a condition in which a large group of people exhibit the same state of violent mental agitation