This case study is taken from Enoch and Ball's 'Uncommon Psychiatric Syndromes' (2001, p181):
Various sub-classification of folie à deux have been proposed to describe how the delusional belief becomes to be held by more than one person. For example, folie imposée is where a dominant person (known as the 'primary', 'inducer' or 'principal') initially forms a delusional belief during a psychotic episode and imposes it on another person or persons (known as the 'secondary', 'acceptor' or 'associate') with the assumption that the secondary person might not have become deluded if left to their own devices. Folie simultanée describes the situation where two people, considered to independently suffer from psychosis, influence the content of each other's delusions so they become identical or strikingly similar.
Folie à deux and its more populous cousins are in many ways a psychiatric curiosity. The current Diagnostic and Statistical Manual of Mental Disorders states that a person cannot be diagnosed as being delusional if the belief in question is one "ordinarily accepted by other members of the person's culture or subculture" (see entry for delusion). It is not clear at what point a belief considered to be delusional escapes from the folie à... diagnostic category and becomes exempt because of the number of people holding it. Whilst large number of people may come to believe obviously false and potentially distressing things based purely on hearsay, these beliefs are not considered to be clinical delusions by the psychiatric profession and may be labelled as mass hysteria.