Four objections:
1) In the Return of the Living Dead movies, zombies can be dismembered and still function. Also, flesh isn't needed; even a skeleton can animate. Remember the woman zombie with its spine twitching, tied to the table? So it looks like, at least for those zombies--the result of a toxic chemical--you'd have to wait until bone rots for them to stop moving and attacking, and that takes decades.
2) In the Romero movies it's not biting that starts the zombie outbreak and it isn't biting alone that passes on zombiosity; it just happens when you die during this very weird and bad time (in addition to biting in Dawn, although not in the original Dawn, I believe), as it did to Ben Cozine (1990 remake of NoTLV). It comes down to: God did it. The recently deceased--and many NOT recently deceased--re-animate regardless of whether or not they're bitten. So as people die--and we all die--they become zombies. It's never clear when this divine zombification ends, but in Dawn and Day it sure looks like it doesn't end until humanity is toast.
3) Zombies attack in hordes; bears and lions don't (well, lions might attack a groups of 3 or so). A horde of zombies can bring down a bear. I really wouldn't look to large predators (not existing in any great numbers anyway) for help. They seem to be immune to zombification, but they're not immune to being bitten and ripped by 50 or 60 zombies. So what if a grizzly wipes out 40 out of 60 attacking it? The same thing that makes us a big problem for big predators would make zombies an insurmountable problem for big predators: sheer numbers, billions to 10s of thousands.
4) I don't see the CDC doing anything about the Obama zombie outbreak. Is Obama "zombie zero"? Anyway, so much for the emergency medical community.
