Most varieties of the flu go through a lot of changes, mostly because even one infected individual sheds so many millions of virus particles, and given just the odd virus replication error or combining with another virus infecting the same cell, there are lots of possibilities for changes in the virus. Some viruses evolve like that faster than others but the flu tends to be one of the faster ones.
Obviously nobody can be certain whether a human-transmissible bird flu virus will evolve as quickly as other flu viruses or if it will be relatively stable but he's guessing that it will be like other flu viruses in that respect - which is probably a pretty safe bet.
It's hard to say for sure if the virus has been tinkered with by man, but if it came from some terrorist's lab it sure seems like they'd have tried to make it transmissible between humans before releasing it. My guess is not - usually infectious biological agents make very dicey weapons because of the very high likelihood of "blowback" into the originator's country.
And I should have added that that's why there are so many new varieties of the flu every year.
Thank you for explaining why/how the flu mutates.
in the meantime they are over manufacturing the shot we'll most likely never need and $$$ will be thrown in the trash