Can a delusional idea be contagious? This question may seem paradoxical when we know that the very definition of delirium correspond to “an erroneous belief (…) maintained despite the very generally shared opinion” (DSM4) and so implies that peers do not share the beliefs expressed by the subject. However, cases of collective delirium have been described for many years, and have been the subject of numerous scientific publications since the 19th century. Among them, an entity emerged : “folie à deux”, in which a primary active subject could induce his delusions to a secondary subject, more vulnerable, said induced and passive. In 1877, Lasègue and Falret first introduced the term “folie à deux” and proposed the diagnostic criteria. They describe nine essential criteria among which, three would be the sine qua non conditions that can allow the outbreak of a delirium shared by two. They are the presence of an active element of superior intelligence, the existence of a common life between the two individuals, sufficiently long and intimate and a “closed and isolated” environment. These criteria were subsequently supplemented to arrive at the current definitions of induced delusional disorder (ICD10) and shared psychotic disorder (DSM4). This rare disorder has been the subject of numerous publications. However, these publications were often divided over both its epidemiology, its diagnostic criteria, and the specific treatment to be offered. The two recent definitions resulting from current classifications can also illustrate this dichotomy on certain criteria, beyond the very semantics which here oppose the terms “induced delusional” and “shared psychosis”. Moreover, this disorder has the particularity to question its real existence as it is currently challenged in the new classifications of the DSM V and ICD 11. We can therefore see that if the subject fascinates, it divides. What is it really? Can a delirium really be transmitted? Can a psychosis really be shared? And if so, is one of the two definitions more suitable to describe this disorder? What future can we imagine for this pathology? During the hospitalization of a patient for a “délire à deux”, concerning two persons from two different families sharing a delirium of filiation, we observed the current issues around this disorder and we asked ourselves which treatment to administer to these patients. The hospitalization took place over two stages: the first stage to understand the disorder, the second one to treat it. À family interview was conducted in the presence of the dyad of patients, to explore the interactional elements together, and establish the diagnosis. A preliminary step essential to therapeutic work on the question of loyalty and differentiation. This clinical case recalls the value of an integrated approach based on the systemic epistemology, both for the diagnostic phase than during therapeutic support. The objective of this work is to study, through an atypical clinical case and a review of recent literature, the different diagnostic, therapeutic and evolutionary perspectives of this particular pathology.