The 1918 Spanish Flu epidemic seemed innocent, but killed many millions worldwide, overshadowed by the war dead from WWI.
I think a lot of people were more concerned about WWI and the upheaval it caused, than by illnesses. A lot of information regarding Spanish Flu deaths was suppressed and not known by the public until much later, hence my statement that WWI overshadowed the flu deaths. Also, it turned out that many soldiers and civilian deaths attributed to WWI havoc were later revealed to be caused by flu.