The flu was originally only affecting chickens, but then it mutated, evolving it's intelligence to effect Mammals then Humans. The soldiers of World War ii, if they had it had no idea. The Influenza bacteria, makes Gripe ( harms the chest), and takes over the lungs, causing breathing problems, which leads to death."The flu was unstoppable, if you had it you were certain to pass away" Scientists in the University of Princeton said.
The reason for this is that when a flu attacks, the immune system tries to destroy it, this flu however takes advantage of this , because the Immune System is slowing the aggression of the virus, the flu takes it's time, slowly killing the person. This means that the people with weaker immune system has a greater chance to survive the flu. In a way your own Immune system is letting you down.
Fifty to 100 million ( 1/4 ) of the worlds population detracted because of the outbreak and it only took a few weeks.
To get the flu you had to be in close, un-hygienic quarters and through contact.