I was doing some digging on this a few years ago...Spanish flu killed the young and healthy by turning the body's own immune system against itself.  Lungs (and body) are flooded with white blood cells.  Those with weak immune systems lived, the old and very young.  If I recall correctly, the CDC has "remade" the Spanish Flu germ/virus to "study."  I'm sure it's all good tho..(sarc).