As is tradition. The Spanish flu was the American flu.

Within days of the 4 March case at Camp Funston, 522 men at the camp had reported sick.[107] By 11 March 1918, the virus had reached Queens, New York.[108] Failure to take preventive measures in March/April was later criticized.[109]

As the U.S. had entered World War I, the disease quickly spread from Camp Funston, a major training ground for troops of the American Expeditionary Forces, to other U.S. Army camps and Europe, becoming an epidemic in the Midwest, East Coast, and French ports by April 1918

https://en.wikipedia.org/wiki/Spanish_flu