Just before breakfast on the morning of March 4, Private Albert Gitchell of the U.S. Army reports to the hospital at Fort Riley, Kansas, complaining of the cold-like symptoms of sore throat, fever and headache. By noon, over 100 of his fellow soldiers had reported similar symptoms, marking what are believed to be the first cases in the historic influenza pandemic of 1918, later known as Spanish flu. The flu would eventually kill 675,000 Americans and an estimated 20 million to 50 million people around the world, proving to be a far deadlier force than even the First World War.

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The initial outbreak of the disease, reported at Fort Riley in March, was followed by similar outbreaks in army camps and prisons in various regions of the country. The disease soon traveled to Europe with the American soldiers heading to aid the Allies on the battlefields of France. (In March 1918 alone, 84,000 American soldiers headed across the Atlantic; another 118,000 followed them the next month.) Once it arrived on a second continent, the flu showed no signs of abating: 31,000 cases were reported in June in Great Britain. The disease was eventually dubbed the Spanish flu because people erroneously believed Spain was the epicenter of the pandemic.

The flu showed no mercy for combatants on either side of the trenches. Over the summer, the first wave of the epidemic hit German forces on the Western Front, where they were waging a final, no-holds-barred offensive that would determine the outcome of the war. It had a significant effect on the already weakening morale of the troops—as German army commander Crown Prince Rupprecht wrote on August 3: poor provisions, heavy losses, and the deepening influenza have deeply depressed the spirits of men in the III Infantry Division. Meanwhile, the flu was spreading fast beyond the borders of Western Europe, due to its exceptionally high rate of virulence and the massive transport of men on land and aboard ship due to the war effort. By the end of the summer, numerous cases had been reported in Russia, North Africa and India; China, Japan, the Philippines and even New Zealand would eventually fall victim as well.

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The Great War ended on November 11, but influenza continued to wreak international havoc, flaring again in the U.S. in an even more vicious wave with the return of soldiers from the war and eventually infecting an estimated 28 percent of the country’s population before it finally petered out. In its December 28, 1918, issue, the American Medical Association acknowledged the end of one momentous conflict and urged the acceptance of a new challenge: fighting infectious disease.