(Covers information from Anderson's universe)
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 large number of ordinary Undead, thus creating the Super Undead. They were all killed by the combined efforts of Alice and the members of Claire Redfield's convoy in a battle in Las Vegas.
The Super Undead, while retaining a mindless hunger for human flesh, displayed a significant increase in their intelligence. The original specimen demonstrated some rudimentary reasoning skills by placing differently-shaped blocks into the corresponding holes. It also displayed memory, correctly demonstrating the use of a cellular phone, digital camera, and one tried picking up a chair to smash the bulletproof glass to escape when they got angry. However, once they get angry, they lose control, as seen during the movie. They are also stronger than regular zombies, as they were able to break free from the restraints that presumably would have held a regular zombie. Also, it seems that they could feel pain and exhibit emotion.
The jump in intelligence was accompanied by an increase in the potency of the Undead'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.