Walkers

Zombies, mostly referred to in-universe as walkers, roamers, lurkers, biters, "'wasted'", and infected are an antagonistic force that serve as the primary catalyst.

Reanimated human beings, while not immortal, will not 'die' under typical conditions that would ordinarily cause the death of a person. They do not appear to feel or respond to pain, can survive even the most brutal injuries, and despite their bottomless appetite for flesh, they do not need food, water, or sleep to survive.

Their functionality appears to center around their brain, as its destruction is the only thing that effectively 'kills' the undead. Even when decapitated, the head will remain active, even though it would be practically harmless at such point.

The Zombies
Zombies are relatively weak and unintelligent as individuals, but are dangerous in large numbers and in tight spaces. They are the main enemies within the post-apocalyptic world of The Help. The majority of known humanity has been wiped out by zombies, which have come to outnumber humans 5,000 to 1.

As a species, zombies do not evolve and are permanently doomed to just deteriorate until there's nothing left but the skeleton.

Reanimation
The dead corpse of anyone that dies for any reason will reanimate as a zombie, unless the brain of the individual is badly damaged or destroyed or the person was dead prior to the outbreak. When a person dies, the infection they carry reactivates critical areas of the brain that support necessary vital systems, resulting in reanimation. Because only a portion of the brain is reactivated, the reanimated person retains only a physical resemblance to their former self.

A corpse can reanimate between three minutes and eight hours after death, though the video game suggests that it could happen in seconds.

The first cases of infection progressed through a state of fever, aches, and internal bleeding, and this illness ultimately was fatal. The pathogen spreads into the brain like meningitis. It infects synapses, mostly concentrating on those in the brain stem. It eventually causes the adrenal glands to hemorrhage and the brain to shutdown, all brain activity would cease, followed by the major organs and the body would be clinically dead: no measurable brain activity, no reflexes, and no respiration or pulse. A variable time later, the pathogen, through some means, would revive synapses it infected and reactivate the brainstem and other parts of the cerebrum and cerebellum of the dead body.

It has been demonstrated that zombies don't require sustenance by eating, but have a strong desire to do so. This is despite the fact that they have no digestive or circulatory activity which makes them unable to digest whatever flesh they consume. Zombies do not need to breathe, evidenced by Pete Dolgen still trying to reach for humans while underwater.

Zombies may very rarely "dodge" melee attacks by leaning out of the way slightly, and some have been observed holding up their arms to likewise block attacks.

Zombies are slower than humans.

Behavior
A reanimated body responds to stimuli such as light, scent, and loud noises. Oddly, even if the head is separated from the body, as long as the brain is intact, the head will still attempt to eat anything within reach. The body of a zombie does appear to be truly dead, which means that it does not feel pain, has no reflexes, and wounds to it will not heal; its rate of decomposition slows drastically but does continue.

There is anecdotal evidence that some retain vestigial elements of memory and personality and this is shown some repeating behavior such as clinging to possessions, attempting to open doors, and even using large rocks to break through windows and doors. Zombies though, are incredibly unintelligent and generally unable to use tools or understand the need for them. It has been hinted at that newly reanimated corpses retain some basic sense of intellect and perhaps memory, and can perform very basic tasks such as opening doors. They have no sense of self-preservation other than eating, and will not react at all to the deaths of other zombies or to potentially lethal dangers to themselves.

Zombies instinctively bite whatever prey they come in contact with, but have also been observed clawing at, tearing, and even punching humans and animals in order to topple them.

Infection
The zombie pathogen itself is not lethal, and the zombie "infection" occurs due to pathogen weakening the host immune system. This makes bacteria found in zombies, especially in their mouths, that much more lethal than they normally would. Nevertheless, the pathogen has two separate but parallel modes of infection - latent and fluid contact/bites/scratches.

Disposal
Because a zombie is derived from a human form, it is limited physically by many of the same constraints that a human has. But because a zombie is, by definition, dead, it slowly rots and decays like other dead organisms. Over time, flesh and muscles deteriorate, and it becomes less and less formidable.

They are only truly killed by destroying the brain, a common theme in many zombie variations. A severed head will remain animated and aggressive until the brain is destroyed or eventually disintegrates from decomposition. Fire has little effect on zombies, other than possibly angering them further, and normally lethal things such as acid or electricity also do little to impede them. They can be paralyzed if their spinal cords are severed, though this does not kill them, even if their heads are severed from the bodies

Intelligence
Though zombies retain a physical resemblance to the living, cognitive similarities are almost non-existent beyond low-level functions, though there are examples of behavior that suggest zombies may retain small fragments of memory of their past lives. Zombies have enough intelligence to walk upright, to use their bodies to break objects, and to climb around or over somewhat small obstacles such as chain-link fences. Zombies can ascend and descend staircases, albeit very clumsily and slowly. They cannot generally operate doors or gates and only attempt to do so when they are relatively "fresh"; they tend to bash through obstacles rather than traversing them.

Mostly, zombies are void of any emotional expression and thought. When stimulated, whether by noise, sight of prey, or simply encountering a problem they cannot solve, such as being unable to open a locked door, they quickly descend into a state of murderous aggression. If they spot prey when stimulated, they can pursue them ceaselessly, showing ravenous hunger.

They are not hunters, however, and take no concern in alerting their victims or trying to hunt them with intelligence, always seeming to roar, grunt, and growl whenever they are stimulated. Zombies seem to be drawn to noise, presumably because they attribute the source of the noise to be caused by potential victims.

Zombies also tend to form groups and stick together, and mimic the actions of other zombies, giving them a mob mentality. This can lead to zombies forming "herds," large hordes of zombies are far more dangerous and aggressive than smaller groups. Zombies retain a further animalistic trait within their "herds," appearing to "sniff out" or examine new-coming zombies before leaving them be, as they do to Rick and Glenn, which may indicate that zombies seem to be able to familiarize themselves with their own kind and be cautious of ones they do not recognize.

Newer zombies may rarely use primitive tools, such as using a heavy brick to smash a window, but none have any high-level abilities to use items in their environment.

They lack any remaining speech capabilities, and can only moan, grunt, or wheeze, as well as roar and scream when alerted. Zombies are not shown to be able to "communicate" with other zombies by any means, though they will frequently copy the actions they witness other zombies perform, such as bashing on a door or moving in a group, eventually forming a herd.

Eating Habits
Zombies prefer to eat living flesh: animals and people. Zombies do not digest food. When their bodies are "full," the undigested meat will be forced out through the anus. As zombies are dead, their bodies will continue to rot even if they are well-fed.

Zombies are never shown in any media to exhibit cannibalistic tendencies, even after going through long periods without food, and only show interest in animals and living humans.

If the zombie loses the ability to feed, they evidently lose the desire to do so - a behavior observed in Walkers. With their loss of desire for eating also comes with a loss of aggression and activity; unless present around other zombies, those who have lost the will to eat will remain quiet and lethargic, mindlessly walking in any given direction and paying no mind to humans.

The presence of many zombies being partially consumed or missing limbs also indicates that zombies, though they seem perpetually hungry, do not always devour prey fully, meaning that, at least for a short period of time, can feel "full" and not want to eat.