Zombies must have a half-life. They exibit signs of decay as soon as they transform into the undead. Their rotting flesh would attract decomposers, like flys and beetles. Those insects should be able to quickly break down rotting zombies and reduce them to bone through the action of their maggots, but they don't. Why do the Walking Dead zombies stop decomposing?
It depends on what universe you are in.
In most cases, the answer is the rather unsatisfying "it's magic." Whatever mystic force re-animates the body of the zombie also halts the decomposition process, the same as it does for other undead like vampires.
In some universes (I believe the Dresden books are like this), nothing stops them from decomposing. The zombies continue to deteriorate even as they are reanimated, until they become almost useless. Of course, a sufficiently strong magic user could bind the bones together in the absence of flesh, or even provide a magical substitute given only, say, the skeleton of a Tyrannosaurus rex.
As far as The Walking Dead universe, to date I don't believe either the show nor the comic have revealed what exactly makes a human become a zombie, other than the fact that any cause of death results in a zombie. However, according to the Walking Dead wiki:
Despite still moving, zombies still decompose as regular corpses.
It also lists specific cases where a zombie was too decomposed to stand/act/attack a human, speculating that eventually the zombies will all rot away and effectively end the infection (and human life).
I was giving this some thought earlier today after watching the end of Season 2.
Forensic anthropologists will tell you that the human body decomposes in roughly one week when exposed to air or water. Temperature and humidity affect this timeframe: hot/wet will accelerate decomposition, whereas cold/dry will delay the process.
As most people are aware... Georgia (and most of the Southeastern USA) gets extremely hot and humid throughout the summer months. A human body in that environment would decompose in under a week. Most of the zombies in the show should have rotted away long before the end of Season 2 (as winter is approaching).
We have to assume that the "zombie virus" somehow prevents decomposition (or drastically reduces its speed). Decomposition results from little critters (primarily micro-organisms) digesting the organic material. They cannot do this to living things, because we have immune systems which thwart those creatures before they start.
It's conceivable that the zombie virus itself behaves like a rudimentary immune system -- whereby by some chemical or biological process it prevents the body from decomposing entirely.
At the end of the day..... it's science fiction and we have to suspend our disbelief if we're gonna enjoy the show :P
Robert Kirkman has stated unequivocally in the letters column of The Walking Dead comic that he has no intention of defining the cause of the outbreak. He's also stated that he's mostly following the George Romero-style zombie rules. Since we do not know how it started, and never will, all answers to this question will be speculation. There is no way of knowing exactly what's going on inside the zombies or why they are animate. Plus, the rotting flesh would not necessarily attract flies and beetles, etc. The virus/plague/whatchamajig may deter those things. Zombies are not typical decaying matter. There's something "wrong."
Perhaps the microorganism causing a zombie's reanimation is not a virus but a fungus. Fungi can get energy from decaying matter, and many of them are also bactericide, vermicide, etcetera. So, maybe a zombie is just a fungus that uses a human corpse as a vehicle to move around and extend its DNA in a faster way. It somehow stimulates the hunger in the zombie's brain in order to get itself a source of decaying meat from which to extract energy. So, if the zombie does eat, the fungus feeds on the content of his/her stomach; if the zombie does not, then the fungus has to feed on the zombie's rotting body itself. While, scavengers like insects, other fungi and bacteria are repelled by the z-fungus.
They don't stop decomposing. In The Walking Dead, unlike the book World War Z, zombies steadily decompose, albeit rather slowly. In the book World War Z, the process still takes place, but usually at a far slower rate (the exception being the rare cases in which the zombie is in an extremely humid, hot environment, like a jungle; in this scenario, the zombie will decompose very quickly).
Getting back to TWD, the fact that the zombies decompose is particularly noticeable in the cases of zombies who have spent some time in the water. Their bodies become bloated and water logged, and their tissue becomes quite fragile:
The production team has also explicitly stated that as the seasons progress, they have tried to make the zombies appear more and more decomposed and emaciated. I have to admit that, in my opinion, they have been extremely inconsistent in this regard, but they claim to have taken it into consideration, and tried to reflect the fact that time is passing and the zombies are becoming increasingly decrepit.
In the "Letter Hacks" column in issue 121 of the comics, Robert Kirkman shed some light on this issue, but not much:
It was most apparent in season 2, in which you really can see how skinny most of the zombies are. The casting director specifically selected people who were extraordinarily thin, and if I recall correctly, they had a lot of success in hiring marathon runners for exactly this reason.
Robert Kirkman has also said that as time goes by, the zombies lose much of their mobility, speed, and strength, as well as their already-meager amount of intelligence. This is actually how he explained the fact that in season one, and only season one (in fact, only the first two episodes of that season), zombies do things that seem to be beyond their capabilities - they almost run in some scenes, they use rocks to break windows, Morgan's zombie wife tries to turn a doorknob, and one little girl zombie stops to pick up her teddy bear. According to Kirkman, all of this is explained by the fact that the zombies are much more intact and undecayed, relative to the zombies of later seasons.
From a Reddit Ask Me Anything with Robert Kirkman:
Q: "...In the beginning of the show we saw walkers do things like using a rock to help bash the doors in or turning a door knob, is there a reason we've stopped seeing them do that?"
A: "Older zombies are less together and capable or doing things like that. Fresher zombies, which there were more of in season one, are able to do more than older, more rotted zombies."
I have been obsessing over this since season one. Remember, the zombie heart is not beating, food is not digesting, so anything in their stomachs sits and rots away. The CDC scientist stated that the primitive brain reanimates and gets the dead up and about with a desire to eat living flesh. We can see them take shelter in a bus in Atlanta in the first season, looking like they are sleeping until they hear Rick coming by on his horse. However, normal time or not, they are decaying. So, after a hot Atlanta summer, we would expect alot of the zombies to be no longer capable of mobility.
I think eventually the zombies will decay and fall apart, but the virus itself is significantly delaying the decomposition process. The zombies from the show have varying stages of decomposition so it's just a matter of time before the micro-organisms win the battle and consume the organic material.
In The Zombie Survival Guide, it is stated that Solanum, which is the virus that creates zombies, functions as a preservative like formaldehyde, and gives them an estimated shelf-life of three to five years in normal conditions. Extremely dry or cold conditions could lengthen this, while extremely wet conditions would shorten it.
Simple answer: everything dries out; the sun is relentless. All that zombie slobbering is losing moisture. In a week or less they're dried out, immobile, brittle. Simple thermodynamics in any universe. Game over.
Near immortal zombies are a cheap plot complication just like never ending bad guy wrestlers.
Their rotting flesh would attract decomposers, like flys and beetles. Those insects should be >>able to quickly break down rotting zombies and reduce them to bone through the action of >>their maggots,
Perhaps the fact they are still moving drives flies away?
I would guess as well that flies can detect they are not normal (animals seem to be able to and will bark, run etc whereas they would ignore an actual corpse) and shy away from them.
The grand-daddy of Zombie fiction is Richard Matheson's "I Am Legend." In this book, protagonist Robert Neville discovers that the zombies secrete a glue-like substance that blocks air from reaching their tissues, thus slowing decay. (Neville viewed the walking dead as "vampires" rather than zombies, but their shambling, mindless description informs later zombie fiction. In fact, the Walking Dead's Dr. Jenner is likely a nod to the way Robert Neville has been depicted in films like "Last Man on Earth" and "Omega Man.") This glue would seal an ordinary puncture wound, but could be ripped open by a large incision, such as a machete or sword would inflict.
This is a very workable explanation. The viral fluid secreted by the zombie infection seals the zombie's system against decay by air and may work as a sort of bug repellent or pesticide. Those zombies who appear in an advanced decayed state may be zombies who were injured in some type of confrontation, thus ripping their flesh open and exposing their anaerobic inner workings to air, thus allowing a rapid decomposition.
Your question is why do the zombies in TWD stop decomposing... Few things you need to know about decomposing:
What stops decomposing? obvious something that lives. but its not the living that stops decomposing, you and me are decomposing at this very moment, but the living part of us is regenerating all used cells, so we can live on ( stem cells are much less active at metabolic level then for example liver cells, and can supply new cells for long periods). That explains why we get older, the cells replenishing the used cells are getting weakened too, and so our regeneration slows compared to the decomposing rate. Do note that i didnt explain different celltypes ex. neurons ect, but that would be too long too explain all.
Now on to the dead tissues! Several organisms are known that slow decomposition by preventing bacterial growth such as different types of fungus ( anti-biotica has been pure killing fungus and extracting their bacteria killing juices at the very start of anti-biotica use).
But there come the flies and other insects, but... as shown in the last half of season 1, when rick and his group stayed at the CDC, you saw that the virus reactivated the brain. A virus is defined a virus by a few traits:
it is not living itself,
it has an ''envelope'' where the viral RNA ( almost same as DNA) resides,
and needs host cells to reproduce e.g. they invade host cells, inject their RNA into the host DNA, changes cell singalling and the host cell starts to produce viral RNA, and packs them in envelopes, and in turn this production kills the host cell, releasing the virus, and the cycle starts again.
In a certain way this is some form of living, since ppl at this field agree that virusses are a most potent entity, since it alters host DNA for the good or bad. Maybe it alters host cells so that it seems living to some degree, hence shuffeling around and growling zombies, but no flies or bugs because they see the zombies as living.
This is just my thought on why they wont decompose to find peace with it all and just enjoy the show. I am sorry if i bothered any one with too much deep stuff that i got from my current study.
In Letter Hacks #14 and #111 it's stated Zombies eat for impulse and posess no functioning digestive system, with what they've eaten being forced out once its full, unprocessed. It's not required for their sustained animation.
Robert Kirkman wrote on Reddit:
...In the beginning of the show we saw walkers do things like using a rock to help bash the doors in or turning a door knob, is there a reason we've stopped seeing them do that?
"Older zombies are less together and capable or doing things like that. Fresher zombies, which there were more of in season one, are able to do more than older, more rotted zombies.
The worse state of older zombies holds the key; when a human starves, the body will break down fat, then muscle, followed by vital organs. The slide in mental function seems to imply the parts of the brain stem re-animated are canibalized in the process.
Since zombies do not digest, this would be how they survive, and the slower decay that stops them being a bare skeleton in a matter of a few weeks,(#9 and #11) could easily be the virus' way of preserving and stretching its 'food' source.
They would end up running out and 'starving' (in a manner of speaking), but there is no indication of how long, other than that that a surviving human would most likely outlive it, through the years, and that a zombie's lifespan would differ depeding on the individual, (#7); which makes sense, given body fat ratio, muscle density, state of organs and tissue changes from person to person.
The reason the dead can rise is because when one of them bites you the virus they inject into you goes to your brain and creates a sort of larva that will breed and die and when they die they give off protein that the brain needs to keep going of course the larva are extremely small but there are over at least a million at once and they only have enough protein to power the brain and some off it's functions such as walking grabbing and eating