My grandfather was in the Army in Newport News, preparing to ship out for WWI when he got the Spanish flu. The war was over by the time he got over it. Didn't I read where that was one of the reasons the war ended? So many soldiers were getting sick and also spreading the disease.
That could be, but the main reason was sheer exhaustion of the German side. After Lenin was smuggled into Russia and a separate peace was concluded, the Germans were able to bring a great deal of force from their eastern front to bear against the struggling western allies. The addition of US troops under the brilliant leadership of "Blackjack" Pershing was just enough to tip the balance.