I'm not sure they had that technology in 1918. I read an article about it once I think it was in National Geographic. Many soldiers had this flu and brought it back with them. There is something about war that seems to facilitate sickness and the likelihood of pandemics, epidemics. People are not so healthy...people are sick, hungry...people are on the move and spread the disease during wars.
it coincided with WWI and hit soldiers very hard on both sides. alot of military draftees got killed by it given the close approximity in recruit houses.
actually, germany was the first to use mustard gas, using it on the russians first.