How big was the Spanish Flu? It basically ended World War One having caused so much chaos on both sides of the trenches, the war was just sort of called off.
The Germans last great hope for victory was defeated in the summer of 1918. Starvation due to the allied blockade, new technology called the Tank put the allies at an advantage. Germany's allies was beaten.
The flu happened in the fall.
The big problem with the Flu ended the war theory is just how much it was never mentioned by the leadership after the war. I don't recall any mention of it in "All quiet on the Western Front" nor have I ever read any history of the War from that era saying much about the Flu.
The 1918 Flu only became a big deal recently