This is discussed in several sources, but this is the one I had most readily available:
"In a month or two everyone outside of Spain was calling it "Spanish influenza," not because it originated there, but probably because Spain, still a nonbelligerent, had no wartime censorship to keep its health problems secret from the world. An estimated eight million Spaniards caught flu in May and June. The Spanish claimed that it had come from the battlefields of France, blown over by the strong winter winds, and that it would have been even worse but for the snowy Pyrenees."
America's Forgotten Pandemic: The Influenza of 1918 by Alfred W. Crosby
The latest edition of Ronan Farrow's Catch and Kill podcast describes how NBC agreed to censor his story on Harvey Weinstein. [https://podtail.com/podcast/the-catch-and-kill-podcast/episo...]
Also in the book - the "spanish" flu was called that since Spain was the only place with a free press at the time (e.g., US press was not free at the time) + massing of young men in US in camps - like Ft Devens. Flu traveled from overcrowded camp to camp and then jumped to local populations along rail lines (and then overseas as troops were shipped out) He describes it well, though would make a great visual as it moves from Boston to Chicago to NYC back into Providence/Brockton (which are just south of Boston). Transmission along rail (and shipping) lines.