Mass Psychogenic Illness

Mass psychogenic illness, also known as epidemic sociogenic attacks or mass hysteria is a epidemiological category used to describe occasional events in which many members of a community exhibit common symptoms with no evident physical cause. These episodes occur all over the world. One of the oddest examples of this phenomenon is koro, in which people are "overcome with the belief that his/her external genital—or, in female, nipple—are retracting or shrinking, with fear of that the organ will disappear."

If nothing else, the phenomenon illustrates that shared information need not be good information. If people can convince themselves and each other that something which is observably false is true, then how likely is it that they can convince each other that something unobservable it true whether it is in fact true or not!