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Wikipedia
A Crimson Head is a type of zombie (officialy named the V-ACT by Umbrella). After the T-Virus infects a normal human being, it rapidly initiates a brain cell decline, but should the new zombie be incapacitated, after a time, its cell tissue regenerates and becomes more fresh, making it able to run. The T-Virus causes deeper necrotizing effects and terrible blood seeping, giving it a crimson skin, small claws, heavy resistance to common firearms, slightly increased intelligence and a more aggressive attitude. Capable of a crouching sprint, clawing attacks, and limited jumping, a Crimson Head presents a significant threat to all but a heavily armed combatant. Essentially rendering sidearms useless, engagement with anything but an explosive or shotgun is highly discouraged.
The process is only halted by incinerating the fallen zombie, decapitation or by blowing out its knee caps, thereby destroying any possibility for regeneration and mutation. Areas in which a zombie was dispatched and left untended are to be deemed extremely hazardous, and any encounter with a Crimson Head where the combatant does not have access to more powerful weapons (I.E. Grenade Launchers, grenades, assault rifles, shotguns) are under advisement to flee immediately. If the Crimson Head is killed, it will no longer resurrect itself.