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 mouthes. The most effective methods of killing them permanently are severing the head or inflicting massive trauma to the brain.