Crimson Head

When a zombie has been incapacitated for some time, a process occurs in which the t-virus will begin to run rampant through its systems, initiating a system known as V-ACT, more commonly known as a Crimson Head due to the color of its skin when in this state.

Biology
When V-ACT occurs, the blood begins pumping at an alarming rate, causing noticeable blood seepage in the skin and giving it a crimson shade of red. The subject's fingers and toes also develop sharp claws as well as having an increased muscle mass. The subject's level of aggression has also greatly increased from that of the typical zombie. When it has entered this state, virtually nothing will stop it from pursuing its prey. The Crimson Head even jumps at its victim due to an increase in blood-lust. If a Crimson Head should remain active, specifically when taking minimal to no damage, it is possible for it to continue its mutation, changing to a Regis Licker, before finally entering its most dangerous stage as a Licker.

Attack Patterns
A Crimson Head will "play dead" sometimes until a player walks near it, it will then lnge quickly to stand. A Crimson Head attacks with much force, running after its prey. It runs in a hunter-like posture. It has been known to even kill zombies and other creatures that get in its way. It will sometimes bite causing minor damage or when it gets the chance it will swipe with its large claws causing massive damage. Crimson Heads can also withstand more damage from some firearms than a normal zombie.

Tactics
A zombie can be prevented from turning into a Crimson Head by either destroying the head or its kneecaps while it is still active, or burning the body using the Fuel Canteen when defeated, also while using Jill it is possible to prevent zombies turning into Crimson Heads by burning them with the Grenade Launcher's Incendiary Rounds.

If a Crimson Head should become active, recommended weapons to use against it would be the Grenade Launcher's Acid rounds or a high-powered calibur Magnum gun as both can kill a Crimson Head in one shot (no more than two shots are required).

The First Crimson Head
According to Umbrella Reasearch notes via V-ACT file, the first Crimson Head, via Crimson Head Prototype 1 that was produced was a prototype B.O.W in a dormant state was reconstructed by the t-virus. When it finally became active, it killed many researchers during it's observation. Umbrella Researchers were forced to have the creature put into a dormant state again (since disposing this specimen was not an option), isolating it and conceal it in the backyard cemetary of the Spencer Estate.

Trivia

 * There are a few zombies that will never become Crimson Heads, no matter how they are dispatched. These are Forest Speyer; the two zombies found in the graveyard behind the entrance hall, and the reanimated cadavers found in the lab.
 * In the Umbrella Chronicles, Jill and Chris witness a Crimson Head, noticing how it is much faster and deadlier.