The Infected are a faction in game and play a large role in the plot. They are infected with a toxin that was supposed to be used against infantry. Instead it worked in reverse filling an infected individual with uncontrollable Rage.
The infected appear to be soldiers of the Alpha Force military, but are rendered unrecognizable by a slew of injuries (either self-inflicted or decomposition). These soldiers are NOT zombies, but regular human beings that can only function to kill anything on sight. Unlike traditional zombies, the infected are also capable of powerful motor control and retain all their combat experience.
At the end of the first Strike Force Heroes, the infected are all cured and return to their normal state, unknowing what happened to them.
The Infected are used as skins for the enemies in Challenge 7: Golf Season, known as "Angry Golfers".