The Somme and the rest of WW1 was a terrible loss of life, but far less well known is that the Spanish flu of 1918 killed 50 million people. (more than 20x as many as the war) It was kept very secret to avoid panic.
http://wwwnc.cdc.gov/eid/article/12/1/05-0979_article
I don't think epidemics can be kept secret, Eddie - old newspapers I've read were full of it. You could hardly miss it if people were dying up and down your street.