mass hysteria

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A condition in which a large group of people exhibit similar physical or emotional symptoms, such as anxiety or extreme excitement.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun psychology 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.

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