The Spanish Flu originated in Tibet in 1917.  As the armies fighting in WWI  the flu spread with them.  When it hit France, it changed its character, becoming deadly.  Nearly half of the American "doughboys" who died in Europe actually  died of influenza, not from enemy engagement.

            Often the victim would be dead within hours of contracting the flu. The virus would cause the body to hemorrhage, the lungs would fill with liquid and the patient would drown in their own fluids.  The worldwide death toll was 20 to 40 MILLION, of which half a million is the estimated U.S. death toll.  Nearly 200,000 of those deaths occurred in the single month of October 1918. Some U.S. death estimates are as high as 700,000...10 times the number of Americans who were killed in WWI.

             It briefly reappeared in March, 1919. This time, however, the world was better prepared and the virus was able to be quarantined. Again it disappeared after  inflicting a rapid death toll.