In his attempts to "domesticate" the massive undead population, so as to allow humans to return to the surface, Dr. Sam Isaacs administered a serum derived from the blood of the Alice clones to a number of ordinary zombies, thus creating the Enhanced zombie, or more commonly known as the "Super Zombie". They were all killed by the combined efforts of Alice and the members of Claire Redfield's convoy in a battle in Las Vegas.
Abilities
Enhanced zombies, while retaining a mindless hunger for human flesh, displayed a significant increase in intelligence. The original specimen demonstrated rudimentary reasoning skills by placing differently shaped blocks into the corresponding holes. It also displayed memory, correctly demonstrating the use of a cellular phone and digital camera. However, once they get angry, they lose control, as seen during the movie. Also, it seems that they could feel pain and emotion.
Infection
The jump in intelligence was accompanied by a large increase in the potency of the zombie's viral loads. Sam Isaacs was bitten by one, and still retained the T-virus, even after a massive dose of the anti-virus, which eventually led him to mutate into the Tyrant.
This is a possible way of bringing the Crimson Heads into the movie series.