Sure, in the comics its explained as a literal wrath of God scenario and in the tv series it seems to be headed the same route, though Kirkman and his people have a penchant for deviating ever so slightly from the comic in that sense.
I just find it interesting, as happened in the back of the pickup truck in S1 E2, how each group reacts and learns "Oh fuck! There was no bite and hes STILL a zambie!" and in the game they do just kind of brush past it.
Highly relevant (starts at 2:20):
My only complaint regarding the zombies is that I'm pretty sure a great majority of them should've decomposed from the weather/heat/general wear and tear by now.
Oh snap, time to school Neil DeGrasse Tyson!
Zombies can move, moan, hear, see, and feel. Which means that the brain isn't the only thing being revived, the brain is just what controls it all. Once the brain is taken over it can start a chain reaction of the other body systems, and the fact that they move is proof of that. Basically, someone is classified as dead when their heart stops beating. However cells can go on living from 6 minutes up to hours depending on the cells. So, theoretically, as long as the zombie virus is fast enough to restart the bodies systems in an allotted amount of time zombies could exist. In fact, they do exist! Just not in humans. If you have time, OMG ZOMBIE SNAILS. Also, the game The Last of Us is basses off of a very real virus that only effects certain ant and spider species.
Now it is a whole other argument as to how long this so called zombie would last, and how it would react to trauma and malnutrition, but the fact remains, that there could be a zombie apocalypse! but it would only last a month or 2
Edit: Also, Tyson said that a zombie virus can't spread though a bite. Not only is that wrong (there are multiple real world viruses that would spread though a bite), but in some zombie lore everyone is already infected with the zombie virus, all the bite does is cause an infection leading to your death and the zombie virus takeover. If a living human bites you it would cause a pretty serious infection if untreated, so who knows what would happen after that person had been eating rotting flesh and who-knows-what-else.
*cough* Lilly *cough*
Technically, Molly and Glenn may still be alive as well, depends on whether or not it is true that the video game story for Glenn is a prequel to him in the comics.
I doubt Lilly got very far. She was always pretty volatile but we saw her slip further and further towards outright insanity. Even if she recovered she'd likely be troubled by guilt and I doubt she'd be able to join up with another group given her attitude. Molly has a greater chance of surviving since she seemed to be doing pretty great on her own before Lee encountered her. Lilly, however, needed her father for stability.