Folie à Deux: The presence of the same or similar delusional ideas in two persons closely associated with one another. Folie: from French folly: madness. From fou: mad. French, à deux: for two. A shared psychosis.

The sharing of delusional (often persecutory) ideas and beliefs by two or more (folie à plusieurs) persons who cohabitate or form a social unit (e.g., a family, a cult, or an organization). One of the members in each of these groups is dominant and is the source of the delusional content and the instigator of the idiosyncratic behaviors that accompany the delusions.

Erich Fromm in The Sane Society: “It is naively assumed that the fact that the majority of people share certain ideas or feelings proves the validity of these ideas and feelings. Nothing is further from the truth. Consensual validation as such has no bearing whatsoever on reason or mental health." 

Just as there is a folie à deux there is a folie à millions. The fact that millions of people share the same vices does not make these vices virtues; the fact that they share so many errors does not make the errors to be truths, and the fact that millions of people share the same mental pathology does not make these people sane.