(Covers information from Anderson's universe)
"The subjects have the simplest of motor-functions. Perhaps a little memory, virtually no intelligence. They're driven by the basest of impulses, the most basic needs. The need to feed. "
— The Red Queen describing the characteristics of the Undead.
The Undead are humans who have died and been brought back to life by the T-virus, an engineered virus capable of repairing a cell but which was converted into a military weapon by Umbrella.
On the same vein with their game counterparts, Undeads are the typical result of t-Virus infection on humans. They are distinguished by their cannibalistic urges and active necrosis of the epidermis which make them appear dead. Arguably as result of severe degeneration in neocortex in the brain by the virus, the use of their brain become severely limited that their behaviour is limited to basic impulses such as the need for food and the loss of most sensory perception upon being reanimated. While their obsessive hunger to hunt and kill humans as a food source is still a mystery (as a study by a clone of Dr. Alexander Isaacs suggested they did not even need sustenance at all and could survive for decades), there are possible explanations.
The infection from the T-Virus alters the Undead's body processes, resulting changes such as necrosis of the flesh and bones being exposed in some cases. However, muscle tissue is somewhat strengthened that they can overpower people despite their degenerated motor skills. While the downside of this change are arguably the requirement of massive quantities of meat to maintain (which explain their afromentioned hunger) and over-production of stomach acid, such requirements appeared to be less strict than their game counterparts since they never shown vomiting the digestive substance and still have enough strength to wandering aimlessly for food for decades before exhaustion takes its toll on them that they either died of resorted to cannibalize other Undeads.
As with zombies in game series, Undead's eyes are covered by a white or gray film of mucous either covering the entire eye or just the pupil, as the eye is a sensitive mucous membrane and turns almost entirely into mucous when dead or rotting.
The mutation from their infection also allowed Undeads to survive around conditions that should be fatal to humans that the only way to kill them is through either decapitation, severe brain trauma or breaking their necks. In fact, infected people can still reanimate as Undead as long as their brain and spinal column still intact regardless of how they died. It was for the same reason Undeads can endure considerable punishment and are not felled by singular gunshot wounds as humans can.
Over the course of the global T-virus pandemic, several mutant varieties of Undead emerged, both engineered and seemingly naturally-developing. The Super Undead were created by the cloned Dr. Isaacs as an intelligent variety capable of being domesticated as a labour force to facilitate the re-emergence of the uninfected human race. The research failed when it was discovered that they were liable to violent mood swings and they were deployed instead as military weapons in Las Vegas, where they destroyed in combat with a refugee convoy. "Tyrant" was a one-of-a-kind mutation of the cloned doctor after he was bitten by one of these Super Undead and repeated doses of Anti-Virus failed to cure him.
The "Majini Undead" were a variety with flower-like retractable fleshy tentacles with sharp teeth within their throat. Originally the result of wild, unforeseen mutation of T-Virus (as Wesker displayed similar but more monstrous version of afromentioned ability), this mutation was successfully recreated in a more controlled environment by Umbrella Prime. It was through this new Undead variety that they managed to develop other Undead mutant called Las Plagas Undead and the parasitic organism used to control them and restore their degenerated intelligence.
Las Plagas Undead
The Las Plagas Undead variation developed through the success of recreating mutation of Undead Majinis. With use of Las Plagas, they are repurposed into the facility's security forces and regain their intelligence, enabling them to use weapon and vehicles.
Javo, of which only one is known to have existed, which was killed by Alice at the National Mall in Washington D.C.