Back in the 1918s, there was a flu epidemic called the Spanish Flu or H1N1.  And seeing all of the 1918 dates on the grave stones in the local cemetery I can believe the magnitude of this out break.

Most flu s kills the young, the old and those with failing health.  But the Spanish Flu was different in that it killed mainly the healthy.  It killed by causing normal immune systems to malfunction and attack the healthy body.  Thus, those with compromised immune systems survived while those with healthy immune systems died.

Of course, modern medicine is much different than what it was 100 years ago.  So, conditions what were once fatal in the early 1900s are easily addressed today.