2. World War I helped the disease to spread rapidly
When the disease first appeared World War One was experiencing, for the first time since 1915, extensive movements of troops both at the front and behind the lines. The Americans were arriving in Europe in ever growing numbers, debarking at French ports and assembling at training centers before moving forward to the front. British units shifted in the trenches to accommodate arriving reinforcements from throughout the empire. French troops were doing the same. Nearly all of the troops which had been at the front had been weakened by the poor quality of the rations they subsided on, as well as the harsh conditions of life in the trenches. They had also been subjected to gas and chemical attacks.
Everywhere the soldiers went they encountered overcrowded conditions, poor sanitation and hygiene facilities, and nutritionally poor food. Diseases such as colds and flu of any type thrive in such environments. Doctors faced with the symptoms of the flu were wary of diagnosing it as such, since the symptoms were more violent and varied than previously encountered strains of the disease. Doctors, particularly those at the front lines and in the military hospitals in Europe, were likely to diagnose dengue fever or cholera, and sometimes even typhus was suspected. Because of the wartime censorship restrictions, reports of the disease were suppressed in the media of the time. Spain, which was neutral in the war and not subject to censorship, began reporting of the disease in November 1918, which led to the illness being named Spanish Flu.