The 1918 influenza strain was remarkable in two regards that made it terrifying, beyond simply the death total. First, it had an unusually high mortality rate for an influenza (about 10%, as I recall. This however is far lower than the mortaility rate of such plagues as the Black Death). Second, and unlike almost all other influenza strains, its incidence was not principally among the very young, the very old, and the infirm. It principally struck otherwise-healthy young people of ages 18-30.