The Infected (sometimes referred to simply as Zombies by the survivors) are Left 4 Dead's take on the classic movie and pop-fiction zombie. Rather than being undead, they are living humans who have been Infected by the Green Flu virus, causing massively increased aggression and loss of many higher brain functions such as speech and self-preservation.[1][2] Like cinematic zombies, those present in the Left 4 Dead series attack human victims until they themselves perish or kill their target