I recognize the comic book goes by "everyone who dies becomes a zombie," but the more I think about it, the TV show may not. For example, there have been a number of scenes (Rick at the gas station in the first ep, the traffic jam in this season) where we see dead bodies that don't appear to have taken head shots but also don't reanimate (the corpse Carl lifted the weapons off of in the traffic jam being one example).
On the other hand, maybe the point is that certain things, such as being trapped in a hot car to the point of mummification, can also "kill" the zombies.
