In 2012 I wrote about the impossibility of slow moving zombies that have to bite people to spread zombiism. At the end of Season 2 this problem was solved by revealing that everyone is already infected, so no matter how you die you will reanimate (I'm told if you play with the volume on the Blu-ray of Season 1 you can hear Jenner tell Rick that everyone is infected already). This explains how the zombie apocalypse got going, and gets around the issue of slow and ineffective zombies spreading the infection.
There is still another serious problem that would make the scenario in The Walking Dead impossible, and that is the unrealistic portrayal of the process of decay. Real bodies decay in a way that would make Walking Dead type zombies impossible.
Walking Dead zombies decay to the point where they look scary but don't fall apart completely. They also have plot-specific strenght. In "TS-19" Jenner says that the outbreak started 194 days ago. Season 3 begins about 8 months after this, taking into account time spent at the farm, that means that at least 440 days have passed since the first dead person reanimated. Think about it. You try leaving a body out in the heat and rain, totally exposed for over a year and see what's left. The zombies decay to the point where a fireplace shovel can cut through a skull like a chainsaw through pound cake, or you can pull a face off a skull easier than ripping off a band-aid, yet they never collapse under their own weight, bugs don't eat them, and they can bite through limbs like jello. I don't know about you, but if perfectly healthy and alive me were to bite into someone wearing overalls' leg I would not only not take a huge chunk out of his flesh, I would probably lose a couple teeth, yet these aspic soft zombies can bite through denim and muscle with no problem.
Come to think of it, how do they move if their muscles aren't metabolising anything for energy? If they can somehow break down their own bodies for energy, then what is breaking the body down for energy? The human body works as a holarchy of systems. A severed head cannot survive on its own because human cells are specialised and a head has no means of acquiring oxygen (except the cornea), energy, nutrients, and they cannot get rid of waste on their own. And yet somehow a zombie head can (and must) be able to do all of that. How does it do that? If none of the other organs work how do they move?
A kidney cell cannot just break down its neighbouring cells to get energy. A human is not a colonial organism, individual cells cannot be removed and act as a new creature. Under ideal conditions a whole kidney can remain viable for 30 hours. Inside a warm body in the Georgia summer, unprotected, it will start to decay and be useless in an hour or maybe less. If standing up (these are walking dead), in the absence of circulation the blood will pool in the legs. After a few days some organs have been completely digested by the bacteria living inside them (pancreas, intestines), the body fills with gas as the bacteria expel waste. Totally exposed to the elements, in a warm wet climate, a body will be completely reduced to a skeleton in a few days*, and the bones are often scattered over a large are by carrion eaters, leaving nothing recognisable as a corpse. In a cold dry environment (like in my zombie story, which has living zombies, but there are still lots of corpses lying around) a dead body will last a lot longer, but it's still not immune to decomposition or being eaten. Since nothing seems to eat zombies even though they are free meat, there is still no escaping the powers of decay at the hands of endogenous bacteria.
Under anaerobic conditions (a body with no blood circulation) at least some cells (like muscle cells, which do store chemicals that can be broken down to get energy) can undergo lactic acid fermentation. This produces far less ATP, which is needed to power calcium pumps which allows muscles to relax. Lactic acid messes up the pH of cells and surrounding fluid, calcium ions move around (as they do), causing the muscles to contract. Unable to pump the calcium ions because of lack of ATP the muscles cannot relax. This is rigor mortis. It begins within hours of death and remains for two or three days, when the muscles begin to decompose. The more vigorous the activity a person was in before death (such as running from zombies) the higher the level of lactic acid and the quicker rigor mortis begins. Even with a functioning brain (which is the only "working" organ) a zombie would not be able to move until rigor mortis ceases, so reanimating within a few minutes or hours after death is biochemically impossible. An immobile, reanimated zombie would be completely defenseless for two or three days no matter how fast it reanimated and by that time should have been completely eaten by animals (unless it's inside something where animals can't get it, not even insects). If by some strange quirk of fate a person dies inside a walk-in freezer so not only can no animals eat it, but the bacteria inside won't decompose it either, the water inside will be completely frozen so the reanimated zombie won't be able to move anyway.
The brain is the most power hungry organ in the body, requiring 1/5 - 1/4 of the body's total energy usage (let's say between 200 and 400 calories a day). Even if only the brain stem were active there is no way anywhere near enough energy would ever reach it, even if it began to consume itself by magic, since there is no digestion and no blood circulation. A reanimated brain would re-die very quickly, maybe within hours, maybe, by some magic slow-down process in a few days.
Unless by "zombie" we mean "dead body being manipulated by some sort of puppeteer parasite that can physically force the limbs to move when movement would normally be impossible" or we mean "shape shifting organism that looks like a human body but instead moves by changing its shape" there is literally no way for zombies like in The Walking Dead to work.
I just don't like all the idiots on the Internet who have stupid arguments about what zombies are.
Mike Rotch wrote: "George Romero took I Am Legend, cut out the living vampires, and made the dead ones eat flesh instead of blood and this will forever be what a zombie is even though they're specifically referred to as 'ghouls' in the movie itself, but I am conveniently ignoring this fact!!!!!1"
People who make arguments like that should go back to the basement they only crawl out of at night to buy their crack and RedBull and leave the Internet to sophisticated literati who like to look up cannibal porn. If it looks like a zombie and acts like a zombie it's a duck, okay, whether it's based on vampires or is still alive like in the original meaning.
*Guess what kind of environment Sasquatch lives! Airplanes go down in the forests of the Pacific NW and are never found, what chance is there of finding a dead body in the two or three days before it's rendered unidentifiable. And i̶d̶i̶o̶t̶s̶ ideologues still scoff "where's the body?" You'll never find one. Meanwhile we have DNA from hair samples that's related to but definitely not human, and we have video evidence of a creature with a gait that even gymnasts with decades of experience cannot reproduce, even when in a harness, and dermal ridge patterns that the top expert in the country identifies as being not human and belonging to no other known creature. By this point the only people who say there is not an unknown species of large primate in North America are either 1. ignorant of the data or 2. people who refuse to accept its existence on ideological grounds.