Wikipedia has a good article on the Spanish Flu, but it claims the flu started in SW Kansas (a big avian flyway + pig farming area) and was spread by infected young men heading to military camps during WWI
From thence, around the world
Interesting read
Just a few months earlier, the US military had faced a wave of influenza that struck a base in Kansas, now believed to be an early incarnation of the Spanish flu — so-called because of an assumption, at the time, of its country of origin. It had spread rapidly but was mild, resulting in relatively few deaths before petering out.
One of my mother’s sisters was one of the 675,000 Americans killed by the Spanish flu in 1918. At the time she lived in South Dakota and was 18 years old.