Due to its proximity to army recruit training camps, Fort Ontario was chosen in 1917 for use as General Hospital 5, with construction of 30 new buildings and medical facilities continuing until January 1919. Expanding from 12 to 1073 beds, it became one of the largest army hospitals in the country and was staffed by many highly skilled doctors and surgeons from the northeast. Personnel from Hospital Unit N, recruited from the New York Homeopathic Medical College and Flower Hospital in New York City, began arriving in late summer 1917 and joined recruits already at the fort, forming the nucleus of a staff that grew to more than 800 by January 1919. Patients began arriving in the fall, first from recruit camps in Syracuse and the northeast and later mostly from overseas, with about 1000 treated each month at its peak. The hospital treated around 8000 medical, surgical, and venereal cases, also served as a reconstruction hospital, and about 200 patients died there, including 70 during the Spanish Influenza pandemic of 1918-19. In addition to hospital work, field hospital units, ambulance companies, base hospitals, and replacement personnel trained at the fort for overseas service. Civilian welfare organizations, civilian women providing physiotherapy and occupational therapy, and the Fort Ontario band all supported care and morale. On September 1, 1919, Fort Ontario ceased to exist as a general hospital.