The reason you see the trend in 20-40 yr olds becoming more ill to the virus is the same reason most deaths in the 1918 pandemic was middle age people. The reason is that the H1N1 virus would pretty much make you own Immune system attack your body causing holes and damage in lung tissue allowing fluid to fill your lungs and you would then drown and die. So because this virus would make you own immune system attack your body the strong the immune system you have the worse your going to get. Thats why young and old people was not effected as bad there immune system was not as strong there for it could not do as much damage to your body. I doubt this will get as bad as the 1918 flu and it may not really get much worse than it is now, however then again it may get severe and cause a decent little problem we wont know just have to wait and watch.
As far as the ones of you that believe this is a man made virus that was released, it's possible there is all kinds of possiblities... However I dont think it is, location of origin and the virus itself is not what would be the ideal situation for a human triggered event. It's just another natural flu virus, you people underestimate what a virus can do to ensure that they infect us and do it successfully that is their sole purpose in life and essential to how they survive.