Yes, one theory is that the immune response was overly strong in this robust healthy age group.
The 1918 “Spanish” Flu epidemic, which may have killed 20 million worldwide, is the reason for the big swine flu scare of the 1970s. We’ve known that a killer flu would return, its only a matter of when. This is the reason for the global monitoring of the flu virus and production of the annual flu vaccine. It is also the reason behind the bird flu scare of a couple years ago.
Probably not. The flu virus mutates a lot. Its the reason why the flu vaccine from the previous year won’t help this current year - different strains of virus are at work from year to year.
Actually, swine flu, bird flu and human flu often jump species from one to the other. Its why much flu surveillance concentrates on SE Asia, where humans, poultry and pigs live in close proximity with one another. The 1918 epidemic which killed so many was thought to have crossed all three species, but a military doctor who spent more than a decade reconstructing the virus learned that it only jumped from birds to humans.