Introduction: An unexplained collective illness (UCI) occurred among staff at the Maison des sciences de l'homme et de la société (MSHS) Sud-Est in Nice. The regional office of the French Institute for Public Health Surveillance (InVS) for the Provence-Alpes-Côte d'Azur and Corsica regions (Cire Sud) was asked to investigate this UCI. Methods: The investigation was conducted in September 2013 in accordance with the recommendations of the Institute for Public Health Surveillance’s guide on “Diagnosis and Management of MSDs.” A monitoring committee was established to oversee the management of this MSD. An assessment of environmental studies and an expert evaluation of infectious cases, in which exposure to pigeons was suspected, were carried out. An epidemiological investigation into the sociodemographic and occupational characteristics and symptoms presented by the staff was conducted in collaboration with the occupational health services of the university and the National Center for Scientific Research (CNRS). Results: The infected cases did not have psittacosis. The environmental assessment revealed insufficient air exchange and humidity levels in the premises. Other issues were noted: visible water damage, sewage backflow, proximity to pigeons, and plastic odors. The epidemiological investigation showed that three-quarters of the 88 people who responded to the survey presented with "irritative" symptoms (sore throat and cough) and "toxic" symptoms (fatigue and headache) in 50% and one-third of them, respectively. One-quarter of the staff reported no symptoms. Discussion: This SCI is a sick building syndrome (SBS). The ventilation deficit in the MSHS building appears to be a key factor that must be addressed as a priority. The activation of the monitoring committee, the expert assessments conducted, the communication efforts implemented, and the ongoing work have contributed to resolving the health crisis. Staff involvement, along with the planned work and improvements, should resolve this SBS.