Crimson Head

When a zombie infected with a certain strain of the t-virus has been incapacitated for some time, a process occurs in which the t-virus will begin to run rampant throughout 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 develop sharp claws and an increased muscle mass and movement speed. The subject's level of aggression also increases greatly from that of the typical zombie. When zombies become Crimson Heads, virtually nothing will stop it from pursuing its prey, making these creatures extremely dangerous.

Attack Patterns
A Crimson Head will "play dead" sometimes (much like their zombie predecessors) until their prey walks near it. It will then lunge quickly to stand. A Crimson Head attacks with force, running after its prey. It runs in a posture comparable to the MA-121 Hunter α, though obviously unrelated - the posture is likely due to decomposition. It has been known to even kill fellow zombies and other creatures that get in the way between it and its prey. When it gets the chance, it will swipe with its large claws, causing heavy damage. It will sometimes even bite, causing minor injury. 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, you can burn them with the Grenade Launcher's Flame Rounds). If the player should fail to burn an inactive zombie that has been shot down, after some time (approximately 30 minutes to an hour), the zombie will have mutated into a Crimson Head.

If a Crimson Head should become active, recommended weapons to use against it would be the Grenade Launcher's Acid Rounds, or a high-caliber 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 Research 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 that was reconstructed by the t-virus. When it finally became active, it killed many researchers during its observation. Umbrella Researchers were forced to have the creature put into a dormant state again (since disposing of this specimen was not an option), isolating and concealing it in the backyard cemetery of the Spencer Estate.

Trivia

 * In the REmake, 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 Resident Evil: Umbrella Chronicles, Jill and Chris see a Crimson Head, noticing how it is much faster and deadlier.
 * Encounters with a Crimson Head are seemingly inevitable on at least two occasions: the first being that one is located on the second floor of the mansion (which does not become active until the player retrieves the imitation Armor Key or attempts to set it on fire before retrieving the key.), and the second being the Crimson Head Prototype 1.
 * In REmake, you could use them to knock down Forest Speyer for a bit without making his HE to explote and instantly killing the player.
 * A recent screenshot for Operation Raccoon City shows a item that attracts Crimson Heads, thus confirming their includement in the game.
 * In Resident Evil: The Umbrella Chronicles during "The Mansion Incident part 1" There is a zombie by a staircase. When Chris and Jill get to the top of the staircase it turns into a crimson head and gives chase. Despite the fact you can blow the zombie's head off (Which should've prevented it's change) it's head grows back offscreen.
 * Thers is a small resemblence they bear with the movie-exclusive Enhanced Zombies.