Must read material:
THE ZOMBIE SURVIVAL GUIDE by MAX BROOKS. The website gives a good flavor of the book, which I've almost completed.
Won't help your questions on the movie, GH, since it is its own universe. Zombies are infected with a virus, solarum, transmitted through bites. The virus attacks the brain, elminiating the entire front lobe. The virus only affects humans, so there are no animal zombies.