Zombies are a common threat throughout the Resident Evil video games and other media. In all of the pre-Resident Evil 4 games, the source of the zombies was the Tyrant virus (more commonly referred to as T), a highly mutagenic viral strain engineered by the Umbrella Corporation as a biological weapon. However, zombies themselves are regarded as irregular mutations, victims of unintentional viral infection.
Human beings infected by the T-virus begin to experience severe flu-like symptoms shortly after infection; these symptoms become worse over time and the infectees' skin will decay and peel, and their cognitive faculties will deteriorate until all that remains is a savage, mindless creature that by all rights should be dead. Human T-virus carriers share many traits with the living dead of horror fiction, earning them the unscientific name of "zombie". They have no powers of reason or communication and are driven solely by an insatiable hunger for living flesh, continuously shambling about attempting to eat living people. One bite or scratch from a zombie will pass on the T-virus, dooming the unfortunate victim to eventual zombification. The time it takes for the infection to take hold varies depending on a person's immune system.
Due to rigor mortis, zombies cannot move very quickly and only walk in a slow, shuffling gait. They feel no pain and do not react when injured, completely lacking any sense of self-preservation. Even if a zombie has lost the use of its legs, it will continue to advance by crawling along the ground, using its hands to drag itself towards a potential victim.
Zombies tend to attack by biting and scratching prey, though have also been seen to possess the ability to expel highly acidic bile from their mouths. The most effective means of killing zombies is to destroy the brain, though it is possible to put them down with enough bodily trauma. However, if a zombie is killed without destroying the brain or incinerating the body, it may undergo a secondary mutation.
Below is a list of known zombie mutations caused by further T-virus contamination.