Remember–US forces, due to World War I — were the primary initial incubator and vehicle making the 1918 pandemic spread from Kansas to crowded barracks to ships to across the globe.
The 1918 Spanish Flu started in Kansas. From there it spread to US troops who had been drafted as war slaves to fight in Europe — and from there, the virus leapt across Europe, ravaged Asia — gutted India — hit China (apparently less so than India), burned through Africa, South America, and island-hopped across the Pacific and the world.
The 1918 Spanish Flu sparked in remote Kansas. From Kansas to US troops to war, the virus did what effective viruses do: it spread with superior meta-intelligence.
These days, US troops are the least of problems other than preservation of our forces.