Definition

folie à plusieurs

folie à deux (literal translation: a madness shared by two) n. A condition in which symptoms of a mental disorder, such as the same delusional beliefs or ideas, occur simultaneously in two individuals who share a close relationship or association.

The same syndrome shared by more than two people may be called folie à trois, folie à quatre, folie à famille or even folie à plusieurs (madness of many).
Recent psychiatric classifications refer to the syndrome as shared psychotic disorder.

Source:  http://en.wikipedia.org/wiki/Folie_%C3%A0_deux