Zombies are persistent, tough and frightening. A zombie has lost any sense of reasoning and is driven completely by instincts, most notably the urge to feed. Interestingly, while zombies consume flesh, they don't require it for their survival.
Due to their diet, the body of a zombie begins to over-produce stomach acids in order to disolve the meat quicker. This leads to zombies, in some cases, constantly vomiting the acid.
It is possible that zombies consume living tissue in order to repair their bodies. Nutrients from a newly deceased victim may aid the zombie's body, allowing reproduction of the reanimated tissue. The repairing of the body may reactivate the brain. This theory may explain how Edward Dewey could take more bullets than a normal zombie, upon consuming a recently deceased man. Another example would be the strength of Marvin Branagh after becoming a zombie. However, zombies are often seen consuming corpses of the dead in the games and these zombies are no stronger or more resilient than those which are seen alone in rooms.
A zombie will pursue its target until it is rendered immobile by decapitation, complete destruction of the brain or burning the body. Not being able to feel pain, a zombie can resist several gunshots from a typical small-caliber handgun before falling down, but can be easily killed with either a well-aimed shotgun blast to the head or a high-caliber magnum handgun.
Due to the necrosis throughout their bodies and the degradation of their nervous system, zombies move very slowly and awkwardly, leading to their signature shuffling gait and their easily avoided lunges. However, there are also rare Hyper Zombies (not to be confused with "Crimson Heads") that will approach their prey very quickly and are more resistant to projectile weapons.
A zombie will continue to pursue its prey even after losing its limbs or lower half of its body. Zombies that have lost the use of their legs (either through deterioration or damage) will instead crawl after its prey until it can grab the target and bite into their feet. They can be stopped with one stomp or kick to their head. Other zombies will lay dormant on the ground due to their lack of nutrition, but will bite anything that comes to their direction.
Some zombies are even intelligent enough to pretend to be dead, only to rise up after a determined period has passed. The only way to determine a zombie's death is by checking whether there's a pool of blood under their body. Zombies that play dead can be easily spotted, as their heads usually move in coordination with a character's movements, watching potential prey and waiting for the right moment to attack. This is a basic hunting technique developed within the reptilian part of the brain.
Zombies have been seen consuming the flesh of other zombies, or otherwise infected corpses. This is likely part of the creatures' attempts to gain nutrients. Edward Dewey can be spotted consuming an infected corpse, but upon noticing the living and uninfected Billy Coen or Rebecca Chambers, he goes for him/her instead. This was also observed by Claire Redfield on Rockfort Island when an infected doctor was devouring the infected corpse of his patient (who, upon spotting her, prompted both of them to attack).
A zombie's eyes are covered by a white/gray film of mucous either covering the entire eye or just the pupil. This is most likely because the eye is a sensitive mucous membrane and will turn almost entirely into mucous when dead or rotting. Another theory is the fact that if the zombie's eyes do not produce tears, the lens' of the eye then get thousands and thousands of scratches causing a cloudy effect.
The zombies encountered in the Spencer Estate and in Africa appear to be much weaker and not as phsyically active as previous infectees. They lie on their backs, sometimes moaning or moving slowly. But once a potential victim approaches, they spring up with a grab attack, which can be avoided. Zombies die from any attack, be it from a weapon or melee attack and cannot chase after prey.
As with many classic examples, zombies can be heard making noises such as grunting and moaning instead of using actual speech, due to the degredation of their brains. The moaning and grunting commonly heard from zombies changes throught the series, but some sound effects carry over between games. REmake and Resident Evil Zero all use the same moaning, grunting, and dying sounds. Other games that share zombie sounds include Resident Evil 2 and CODE: Veronica, as well as Resident Evil 3: Nemesis and the first Outbreak.
Variation of infection
Since virtually anyone can become a zombie, they can come in different varieties depending on the location where an outbreak occurs, and the variation of the virus itself. This ranges from ordinary civilians and police officers to the very workers and researchers that were experimenting on the virus. Some zombies were even created within Umbrella's facilities as a result of experiments. Known individuals that have turned into zombies include Edward Dewey; Forest Speyer; Marvin Branagh and Brad Vickers, among many others.
Capabilities of a zombie vary as factors are dependent on the carrier. If infected with certain variations of the t-Virus, a zombie who has been incapacitated for a long period of time can mutate into a "Crimson Head" due to a process called V-ACT. Secondary infection with other variants may create Lickers. Depending on the biological make up of the host it may very well mutate into a Licker or a Crimson Head if completely left alone, but the chances of this occurring are slim. Although judging by the appearance of the Regis Licker, it's possible that the Crimson Head is the second stage of mutation from a zombie into a Licker (albeit a late onset of said mutation).