In early March 1918, soldiers with the flu began reporting to the infirmary at Camp Funston, an army training camp in Kansas. Within three weeks, 1,100 men at that camp had also come down with the flu. It was the start of a pandemic that would kill as many as 100 million people worldwide.
Though commonly called the Spanish flu (because of a highly publicized outbreak in Spain), it likely began in Haskell, Kansas, where it spread to Camp Funston and from there to the rest of the world. Wartime conditions, like troop movements and overcrowded cantonments, accelerated and aggravated the spread of the virus, which proved to be much deadlier than the normal flu, in part because of a particularly tough strain of pneumonia that often accompanied it.
The Spanish flu afflicted cities across the nation and around the world, but since it disproportionately hit young adults in their prime, the military felt its effects strongly. The US Navy would later estimate that 40 percent of its men had gotten the Spanish flu, while the Army reported 36 percent. Of the three waves of the flu (March–June, September–November, December–March), the second wave was the deadliest for both civilians and for the military. In fact, between September and November, the flu killed about as many soldiers as World War I did in that same time period.
The Spanish flu affected the war itself as it ravaged the armies of both the Allies and the Axis. While many soldiers were sick for three days or so and then began to recover, a substantial number either developed the deadly pneumonia as well or contracted a version of the flu that could kill in as little as 24 hours. For every soldier that died, another four or five were too sick for weeks afterward to carry out their duties. Military attacks and operations on both sides had to be postponed because of the huge number of soldiers incapacitated by the flu.
Despite failed attempts by the medical community to control the virus, the pandemic eventually began to die down on its own, with the worst of the third wave finished in the United States by the end of March 1919. By 1920, the danger was finally over.