Mass delusion, mass madness,
"Mass delusion is a false belief shared by a large group of people. In contrast to delusions caused by psychiatric disorders, mass delusions are man-made by diverse influences to promote financial, political or psychosocial goals."
"Shared psychotic disorder (folie à deux) is a rare disorder characterized by sharing a delusion among two or more people in a close relationship. The inducer (primary) who has a psychotic disorder with delusions influences another nonpsychotic individual or more (induced, secondary) based on a delusional belief. It is commonly seen among two individuals, but in rare cases, can include larger groups. For example, it can occur in a family and is called folie à famille."
