The 1918 flu spread rapidly, killing 25 million people in just the first six months. This led some to fear the end of mankind, and has long fueled the supposition that the strain of influenza was particularly lethal.
However, more recent study suggests that the virus itself, though more lethal than other strains, was not fundamentally different from those that caused epidemics in other years.
Much of the high death rate can be attributed to crowding in military camps and urban environments, as well as poor nutrition and sanitation, which suffered during wartime. It’s now thought that many of the deaths were due to the development of bacterial pneumonias in lungs weakened by influenza.