Your grandfather was indeed a typical victim. People over 40, many of them had been exposed to a similar enough virus to have antibodies. Children seemed to have good resistance. But young adults had not. The epidemic went through the Army camps like wildfire.
The Army lost far more men to the influenza epidemic than it had in the Great War.
There are two good books on this epidemic -- one is by the author of this story, Gina Kolata (it's the more dumbed-down of the two -- she was a TV reporter at the time).
d.o.l.
Criminal Number 18F