Guide for the Diagnosis and Management of Unexplained Mass Illnesses The French Institute for Public Health Surveillance, which is regularly called upon by health authorities to assist in the investigation of mass illness outbreaks, has observed that a significant number of these phenomena cannot be explained by a simple mechanism of interaction between the physicochemical environment and biological processes. These types of nonspecific syndromes, which are theoretically quickly resolved—including sick building syndrome—can take on unexpected proportions, particularly due to the psychological and social context in which they occur. Thus, the need for a guide arose to provide health authorities with a method for optimal management of this type of situation, where assessment and management are closely intertwined. The information compiled in this guide aims to assist in diagnosis by considering all factors associated with the onset of these syndromes, including both environmental and psychosocial aspects. It also outlines for scientific and technical experts the key steps of a multidisciplinary and coordinated approach. Its operational objectives are as follows: to promote coordination among stakeholders; to facilitate investigation; to streamline the use of metrology; to assist in effective communication; and to foster a shared culture. (R.A.)