(Covers information from the D.C. comics)
After being injected with the X-Virus developed by Dr. Whitlam, the host goes through a somewhat adequate mutation speed, the person begins to feel a burning sensation in the head. After a few hours, they will begin to mutate, their eyes will turn black and the pupil/iris will be white. They will be growing rapidly in size during this time, as well the hair will begin to fall away, until there is little on the sides and top. They will start acting very primitive, roaring and growling animalistically. On their arm(s) they will sprout claws, much like Tyrants of the past. They will attack almost anything in sight, except for other X-Infected creatures. Their intelligence is more than that of your average Tyrant's. Even though injected with the antidote, it would seem that that's not enough to stop the mutations, as stated by an unknown researcher at the end of Resident Evil: Fire and Ice Chapter 4.