If you wanna go that route, then actually, the Spanish flu is what led to the WW2. ;-)

US President Woodrom Wilson cam down with flu, had a stroke and early retirement from public life. So he couldn't persuade US govt to join the newly formed League of Nations or ratify the Treaty of Versailles. He didn't support punitive measure against German state. Since US was no longer part of process, German paid hard. Hyper inflation took hold, 4marks to a dollar ended up 4 billion marks for a dollar. That hardship and resentment is the environment the fueled Hitler and brought him support. Interesting reading, "The Spanish Flu epidemic and it's influence on history"