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.
https://www.history.com/this-day-in-history/first-cases-reported-in-deadly-influenza-epidemic
Why Was It Called the 'Spanish Flu?'
The 1918 influenza pandemic did not, as many people believed, originate in Spain. The Spanish Flu did not originate in Spain, though news coverage of it did. Because Spanish news sources were the only ones reporting on the flu, many believed it originated there. Allied countries and the Central Powers had wartime censors who covered up news of the flu to keep morale high.
https://www.history.com/topics/world-war-i/1918-flu-pandemic
https://www.history.com/news/why-was-it-called-the-spanish-flu