Introduction: An unexplained mass illness (UMI) occurred among staff at the Maison des Sciences Humaines (MSHS) in Nice. Methods: The investigation was conducted in September 2013 in accordance with the recommendations of the French Institute for Public Health Surveillance’s guide, “Diagnosis and Management of UMI.” A monitoring committee was established. An assessment of environmental studies and an evaluation of infectious cases suspected of being secondary to exposure to pigeons were conducted. An epidemiological investigation regarding the sociodemographic and occupational characteristics and the symptoms presented by the staff was carried out in collaboration with the occupational health services of the University and the CNRS. Results: The infectious cases were not psittacosis. The environmental assessment revealed inadequate air exchange in the premises and insufficient humidity control. 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 staff members who responded exhibited “irritative” symptoms: sore throat, cough (50%) and/or “toxic” symptoms such as fatigue and headache (30%). 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. (R.A.)