Originally Posted by
HDMe2 
Some people mix up their zombie lore.
The Walking Dead is heavily inspired by the original Night of the Living Dead. Kirkman has said before that he envisions his world as taking place at the same time/world as the Night of the Living Dead... and he is just telling the story of a different group of people.
Along that line... the original movie speculated, but did not define, the origins of the undead... they also showed clearly that whatever caused the dead to rise, happened to people already dead... people digging themselves up from graves in the cemetery for example... so there was not a virus killing people and bringing them back to life... but rather "something" causing the dead not to stay dead.
In other words, no matter how you die... you come back.
Now, being bitten by the undead leads to all manner of infections especially with no medical facilities to treat most bites... so a dog bite would be just as fatal potentially!
I haven't read the comic... but working within that framework of Romero inspiration... it seems only logical to me that the dead will return to life UNLESS their brain was destroyed.
So... if you died from a brain tumor, leukemia, or some other brain trauma... you probably wouldn't come back to life at all... anything else and you are back until someone pops you in the head.
A lot of people have been extrapolating theories from things on the show as if the characters on the show know what is going on... so whenever anyone on the show says something, like the CDC guy, some take that as fact as if it was conclusive... but the facts are that very little has been shown on the show... and most of the characters are speculating based upon personal experiences.
Nobody on the show thus far, for example, has seen a person die of non-zombie causes and return to life... so naturally the speculation by those characters will be that zombie bite = infection = return to life after death... but one episode and one person dying of non-zombie method and then coming back would be proof enough that they all were wrong.
I 100% understand the characters on the show not knowing... I don't understand people in the real world watching the show being certain of things the creator of the show himself has specifically said are "left to be discovered"...