I can't vouch for validity of this information but I remember reading somewhere that a team of scientists had resurrected the 1918
fly by digging it out of graves somehow. They were surprised to find a bog standard
fly, no more potent that the strains we have today.
So their conclusion: it wasn't the
fly that was so deadly, it was combination of social factors, poor overall health in the masses and Aspirin poisoning that attributed to out-of-proportion death rate for young males.
P.S. Sorry, I just realized I'm quoting Russian Wikipedia, article
Испанский грипп — Википедия refers to research article
Initial Genetic Characterization of the 1918 "Spanish" Influenza Virus