When humans are infected with the T-virus, their minds and bodies begin to deteriorate. The rate of degradation depends on the subject's immune system, but the result is nearly always the same: carriers are reduced to a corpse-like state, yet the body remains active. The subject's mind is completely gone, replaced by a primal urge to feed on living human flesh. These zombies are slow and stupid, but can be extremely dangerous when encountered in groups and/or in enclosed areas. They attack by biting and scratching their prey, though some have also displayed the ability to expel highly acidic bile from their mouths. The most effective methods of killing them permanently are severing the head or inflicting massive trauma to the brain.
If T-virus zombies are incapacitated without destroying the brain or incinerating the body, they undergo a secondary mutation called a V-ACT. The subject's heart begins pumping at an alarming rate, causing the skin to turn a crimson shade of red. The subject's fingers elongate into sharp claws and their muscle tissue hardens, allowing them to run and granting them superior strength and endurance to typical zombies. These creatures have been referred to as "Crimson Heads" due to the red coloration of their skin. They are much more aggressive than zombies and attack prey mercilessly by swiping with their claws, then eat their victims once they are incapacitated. Destroying the brain is still the most effective way to kill them, but their increased speed and endurance make dispatching these mutants all the more difficult.