They are no longer to be considered Zombies as they have been shown to communicate, form plans, use guns and ride horses as well as reacting to being lit on fire - All things true Zombies do not do.
Process of infection and reanimation
In Romero's Dead series, any human being who dies after the onset of the zombie apocalypse can and will reanimate shortly after death, excluding those who died by massive brain trauma (such as a gunshot wound to the head) or had their brain incapacitated post-mortem. Being bitten by a zombie is not a prerequisite for returning to life, as any deceased human, regardless of exposure to a zombie, will return. No Romero film has definitively revealed the cause of reanimation, but several have featured characters speculating on possible causes, including radiation from a NASA probe, divine intervention, and viral infection. The length of time between death and reanimation seems to vary, but in most instance it is only a few minutes.
If a character is bitten by a zombie, they will become violently ill and die within three days. The interim till death seems to be dependent on the location and degree of the bite (meaning that bites on or near major arteries or veins will spread the infection much faster than small bites or scratches). Multiple bite wounds will make the infection spread all the faster. Also, massive blood loss caused by one of these bites will speed the death of the victim.
In Day of the Dead a limb is amputated and cauterised in an attempt to stop the infection. The effectiveness of this treatment is not revealed, as the character dies of other causes before the infection would have taken effect.
Throughout the Romero films, all zombies are hazardous to human life. Their only instinct is to feed relentlessly on living organisms - they do not feed to satisfy hunger, and even specimens who have been completely disemboweled will still desire to feed, as long as brain function is present.
While most of the personality of the original victim is gone, there have been instances of the zombies seemingly recalling memories of their past lives and performing familiar tasks.
Although initially devoid of both thought and memory, later works of Romero have shown zombies capable of learning through a process of trial and error. Both Day of the Dead and Land of the Dead show zombies using firearms in a limited capacity, while blunt objects as weapons have been used by zombies since Night of the Living Dead. The full extent of the creature's learning abilities is unknown, but the process of discovery for them is not unlike that of a toddler.
The films include a few rare examples of what can only be described as emotion in zombies. However, this emotion has only come after much training, with ample "food" being provided to keep the zombie occupied. A reanimated corpse will attack any and all living humans in its immediate vicinity, regardless of past ties to said individuals.
Romero has stated that the rules to zombie behavior and weaknesses that have become popular with his films and used elsewhere change from movie to movie. In Night of the Living Dead and Dawn of the Dead the zombies are afraid of fire, in the later films they are not. By the fourth film the zombies have memories and communicative skills.
In Land of the Dead the zombies are transfixed by fireworks looking up so people can get close and shoot them, although eventually the zombies stop being transfixed and resume attacking Fiddlers Green.
The movement of all zombies has been shown to be a shambling walk at best (due to bodily decay, rigor mortis, and brain decomposition). Individuals more than an arm's length away from a zombie will easily be able to out-walk one and avoid their advances. However, they have been shown to lunge once in close proximity of a target and will ferociously claw at and bite anything in their grasp. Also, their slow gaits cause them to move extremely silently, allowing them to sneak up on prey undetected. Zombies have been observed using stairs and even ladders, but they usually have trouble with such obstacles due to their lack of fine motor control.
The apparent slowness of zombies has led many unfortunate victims to believe that they could outpace a zombie as well. However, while over short distances a human can outrun a zombie, zombies have almost limitless endurance. A zombie may only be able to move with standard human walking speed at maximum. However, they can maintain such movement for practically indefinite time periods, because they never tire. While a human can outrun a zombie over short distances (provided they keep their distance), the human cannot run forever, while zombies can walk forever, and thus eventually the zombies will run down the human, like wolves descending on an exhausted deer. Physically (that is, bio-chemically), it is a mystery how zombies maintain the energy needed to keep their decaying muscles moving for indefinite time periods. However, this is essentially tied to the overall mystery of how dead corpses are able to reanimate as "zombies" at all, and continue to function without ever actually metabolizing meat from humans they consume.
Physical causes and limits
What actually makes Romero-zombies reanimate is an open mystery. However, causal explanations for several practical affects have been explained. It has been demonstrated time and again that Romero-zombies are dependent upon their reanimated brains. Destroying the brain (or severing the spinal cord and lower brain stem, thus shattering the connection to the rest of the body) will "deactivate" a zombie. In laboratory conditions, zombies have been entirely vivisected, with their entire digestive tract disconnected, but they will continue to try to eat. Further, decapitated zombie heads (still functioning due to their intact brains) will still try to consume flesh. The explanation is that zombies don't actually need to "eat" flesh, in the sense that they don't really metabolize their food. Human flesh they swallow down their gullets doesn't get digested, it just congeals or falls out of them. A Romero-zombie could be thought of less as a whole "animal", and more as a brain connected to a set of muscles, and the remaining sensory organs.
As laboratory tests have proved, the physical reason Romero-zombies behave the way they do is that their brain and muscles have been somehow reanimated, but the brain is gradually decaying and shutting down. When a zombie's brain decays, its internal regions shut down, starting on the outer regions, then working inwards, layer by layer. Essentially, all of the higher brain functions shut down first because they are near the outside (such as the frontal lobe), leaving the more primitive regions of the inner brain intact in zombies. Thus, the remaining functional portion of a zombie's brain are the primitive inner regions containing the most basic and primal urges, inherited from the first amphibians and reptiles: the need to feed.
Consequently, zombies don't actually need to eat (indeed, their digestive systems, even when intact, are not functional). However, what's left of their functional brains thinks they need to eat, filling them with a ravenous, unreasoning hunger.
In laboratory conditions, brain surgery has been used to destroy the inner regions of the brain controlling this primal hunger urge, while keeping the motor skills-controlling sections functional. The resulting zombie subjects are successfully rendered docile by this procedure, confirming that it is the primal sections of the brain which drive their urge to eat. However, this medical procedure on the brain takes about 11 hours of surgery to perform, and only a handful of qualified medical personnel are skilled enough to do it, making it a totally impractical option for dealing with zombie attacks, much less a full-blown worldwide zombie apocalypse.
Why this makes zombies attack humans is anyone's guess. The actual details of what zombies will or will not eat varies from one incarnation to another: in some versions they actually have a preference for only human flesh. However, in other versions, the more congruent explanation is given that zombies will actually eat any "warm meat" that they can actually catch, though dogs and cats (and other such animals) are usually small and fast enough to outrun them. In one strange case, a zombie is even seen eating an insect off a tree in the original Night of the Living Dead.
How zombies can possibly maintain the bio-chemical energy needed to move their own muscles is unknown to science. Consuming no actual food, they physically can't make the chemical energy to keep moving, yet they do. So long as a zombie's brain is intact, and its muscular and skeletal system are still relatively functional, it can keep functioning.
However, while Romero-zombies do necrophy slower than normal corpses, it is stated that after roughly 10-12 years, a Romero-zombie's body will have finally decomposed to the point that it affects the zombie's basic motor functions. After so many years, it has begun rotting to the point where the creature actually falls to pieces. Thus, the final fall-back solution to the zombie-outbreak which totally overwhelms the entire world in the Romero zombie films is simple: the surviving humans should wait in their bunkers until 10-12 years after the initial outbreak, when most humans were infected. By that time, most zombies will have rotted to pieces, leaving only a few stragglers converted from infected survivors caught in breached strongholds.
- Only destroying the brain will result in the true death of a zombie. Decapitation (severing the spinal cord) will only render the body immobile, while the head will still function and is capable of infecting the unwary.
- Although medical treatment of bites has been shown to slow the death and resurrection of victims, there is no known cure.
Zombies based on Romero's works
The zombies from the 2004 remake of Dawn of the Dead bear a special mentioning here. Though they function in (presumably, given the little evidence available) much the same way as Romero zombies, they have a few unique qualities all their own.
- The infection is spread only through bites and scratches. It is not shown whether the "virus" (again the cause is never actually shown, but the DVD case suggests that it is a virus) is passed through fluid contact, but it is unlikely seeing as how on numerous occasions characters are essentially showered with blood and are not infected afterwards. Zack Snyder, the director of the remake, states that the bites from zombies are supernatural, like vampire bites.
- Unlike the original Dead series, simply dying of a non-zombie related cause (i.e. getting shot) will not turn the victim into a zombie. As discussed previously, only bites and scratches will spread the infection.
- People who are bitten by zombies and then finally die transform into zombies within a matter of seconds. Thus, a man who is bitten in the neck and instantly dies of rapid bloodloss, dies and then reanimates as a zombie barely a minute after being initially bitten. In the original, there is some "turnover" lag time of anywhere from several minutes to several hours within which a person that dies from a zombie bite will remain as a corpse, before reanimating as a zombie.
- The most obvious difference in the two classes of zombies are their movement. The original zombies from the Dead series were only capable of walking at a slow pace with the occasional lunge when prey was close. The remake zombies are fully capable of running at top speed. Whether this has something specifically to do with the infection or is just an aesthetic quality introduced by the filmmakers is unknown.
- Romero zombies commonly vocalize with only grunts and low moans. The remake zombies do this as well as emit a piercing shriek or a loud growl when provoked.
- Author Max Brooks gives a comprehensive breakdown of zombie biology in The Zombie Survival Guide. It should be noted, however, that while Brooks was heavily influenced by Romero-zombies, there are some key differences in his works, chief among which is that Brooks-zombies cannot recall any memories of their former lives as humans or learn. Brooks-zombies are utterly mindless and cannot even learn from the most basic trial and error: one character in Brooks' World War Z describes watching a zombie, through a spy satellite video, digging into a sand dune to try to catch a mole that had tunneled in; digging in such loose material only ended up spilling more sand into the hole, so it was physically impossible for the zombie to reach its goal. Nevertheless, the zombie kept digging, almost like a robot, for five days straight, until distracted by other prey. A Romero-zombie in a similar circumstance would probably eventually have realized its method wasn't working. The concept of a zombie recalling how to shoot a gun, which Romero-zombies can occasionally do after much coaxing, would be unthinkable for Brooks-zombies.
- Izombie: In the series, 3rd season, Olivia Moore while watching some jailed zombies with Major and disguised as a cop, states them as "Romero"
- Zombie Types - a comparison of characteristics of Romero zombies and zombies in other films or books.