At a 1944 Nazi bacteriological warfare conference in Berlin, General Walter Schreiber, Chief of the Medical Corps of the German Army told Mueller that he had spent two months in the US in 1927 conferring with his counterparts. They told him that the "so-called double blow virus" (i.e. Spanish Flu) was developed and used during the 1914 war.
"But," according to Mueller, "it got out of control and instead of killing the Germans who had surrendered by then, it turned back on you, and nearly everybody else." ("Gestapo Chief: The 1948 CIA Interrogation of Heinrich Mueller" Vol. 2 by Gregory Douglas, p. 106)