While World War I did not cause the flu, the close troop quarters and massive troop movements hastened the pandemic and probably both increased transmission and augmented mutation; it may also have increased the lethality of the virus. Some speculate that the soldiers' immune systems were weakened by malnourishment as well as the stresses of combat and chemical attacks, increasing their susceptibility. Andrew Price-Smith has made the controversial argument that the virus helped tip the balance of power in the latter days of the war towards the Allied cause. He provides data that the viral waves hit the Central Powers before they hit the Allied powers, and that both morbidity and mortality in Germany and Austria were considerably higher than in Britain and France.

A large factor in the worldwide occurrence of this flu was increased travel. Modern transportation systems made it easier for soldiers, sailors, and civilian travelers to spread the disease. In the United States the disease was first observed at Fort Riley, Kansas, on March 4, 1918, and Queens, New York, on March 11, 1918.

In August 1918, a more virulent strain appeared simultaneously in Brest, France, in Freetown, Sierra Leone, and in the U.S. at Boston, Massachusetts. The Allies of World War I came to call it the Spanish flu, primarily because the pandemic received greater press attention after it moved from France to Spain in November 1918. Spain was not involved in the war and had not imposed wartime censorship.