That really deadly Spanish flu that happened during WW1 didn't even start near Spain. They were just the first ones to publicly be all like "hey, there's something going around killing a lot of people or getting them really sick really fast".
Most likely, as near as anyone trying to track down the origin of the Spanish flu is that it started in a small Kansas farm town and spread from there to the Fulton Kansas army training base, and on over to France.
It killed around 50,000,000 people. Had there been no war, it likely never would have even made it out of that Kansas town with like 1,000 people in it.