As we all know Kirkman has stated that he is borrowing many of the rules and traditions established by Romero. One of the few that he has not borrowed is the whole "learning zombie". The main thing he has kept is the lack of explanation of the origins of the Zompocalypse.
"I can't stand this whole--I don't know, sort of an apology or something that they're not dead anymore in most of these films; that they've got a virus or some kind of rage bug. That's almost like they're apologizing. My guys are DEAD, somebody changed the rules and they're coming back to life."
— George Romero (Lost Zombie interview).
The closest we came to a firm understanding was the main group's visit to the cdc. There Jenner gave his explanation of the cause of it all. He said "it could be microbial, viral, parasitic" when another character offered " the wrath of god" he gave that equal potential.
In the romero movies all anyone can do ( the news, reporters, scientists, characters....and the viewers) is guess.
The whole point is that nobody is supposed to "get it". zombies exist in total contradiction of laws instituted by god and man. THAT is what's scary about zombies. picture a dead body on a table in a room full of scientists analyzing the process of reanimation. and finding nothing. the scientists enraged at the rising corpse because it should not be. a deep recessed part of their brains spasming because of the contradiction of all of the physical laws that they have studied suddenly crumbling to dust in front of them, crying out in denial as the walking horror closes in and devours them as they wail in desperation.
when you try to apply a cause to a corpse rising you cheapen the effect. You are trying to scratch that little itch in the back of your brain that is whispering to you " this can not be, the dead do not rise". You are trying to make yourself comfortable with the idea by assuming that it's a virus.
I think the cdc thing ( wich was'nt in the comics) was a poorly executed way of stating....IT'S NOT A VIRUS. nobody has any idea what it is. the cdc guy lumped in wrath of god as a possible explanation. for simplicity the writers keep saying it behaves like a virus.