It's been a dual-front kind of thing all along. There's the infection they all have that doesn't kill but turns them when they die, then there's the infection that comes from being bitten that kills you quickly with extreme fever. They did show a quick fever when Shane turned even though he wasn't bitten, so it's clear that the fever part actually comes from the reanimation infection even though it manifests while alive when bitten. Makes sense, considering that the virus is also somehow providing near infinite metabolism without bodily functions or food.
That makes no sense, as if you die from natural causes, or gun shots, or anything not directly zombie related, you turn instantly and your bites are then lethal.
These are nothing more than plain ol' magic zombies. The zombies are damn near bullet proof, unless you shoot them in the head! As if some magical spot in the brain is what is causing it. Now, I'd be okay if the virus was something like rabies, enacting only the basic functions of the zombies and crippling most of the nervous system (so they can ignore pain etc), however, removing their heart would kill them. But nope, only headshots!