One disease unique to the First World War was trench fever, or "pyrexia of unknown origin," which was first identified in the British Army in France in the summer of 1915. It had the name because it was "only observed among officers and men living near the trenches, and in the personnel of hospitals, especially among orderlies of wards in which there were patients suffering from the disease." It was a mystery during the war, and Hurst speculated that some insect, possibly lice, was involved in the spread of the disease, as its occurrence was seen least in battalions with adequate provisions for bathing and the incidence of disease could be lessened by measures against lice. Patients could be infected--and infectious--for long periods of time, and infection did not confer immunity to subsequent reinfection. Trench fever also had an incubation period of several weeks, adding to the increased likelihood of spreading infection. The fever occurred in short and long forms, and periodic bouts were common. The disease, though debilitating, was never fatal, and it disappeared with the Armistice.