The 1918 flu that killed MILLIONS was a H1N1 strain. The first wave wasn't bad, but it mutated and the second wave was the bad one. Whenever the H1N1 strain appears, there is the chance that it could mutate to be just as virulent. We were lucky that it didn't. As with the 1918 flu, this H1N1 killed proportionately more young people than other strains.