They have no fine motor control and their gross motor control functions are heavily degraded, thus rendering them incapable of picking up objects and operating simple mechanisms such as door handles. Some Infected suffer severe tissue damage and dismemberment from moderate blunt force, yet they can still jump down from heights of approximately 20 feet with perfect physical coordination and without harm. The Infection might also have changed their physiology. will attack Coach and Ellis even though they will recognize him and call him by his former name.

For example, Carolyn attacked her husband Wade shortly after contracting the Green Flu, Sandra tried to kill Francis mere moments after talking with him normally, Church Guy tried to kill the survivors right after opening a door for them, and Jimmy Gibbs Jr. It seems that psychologically, both Common Infected and Special Infected cannot remember their lives before they turned. However, these attacks cease when the Infected find and focus on an uninfected human.

Common Infected are also frequently observed displaying mutual aggressiveness towards one another. The Infected primarily attack uninfected humans who have attracted attention through noise, movement, displaying bright lights, or anything covered in Boomer Bile (whether it be from a Bile Bomb or the Boomer itself). Bite attacks are of a secondary nature and may even be incidentally delivered. In the cases of Common and Uncommon Infected, observed aggressiveness manifests itself primarily in the form of frenzied physical blows and kicks. Like cinematic zombies, those present in the Left 4 Dead series attack human victims until they themselves perish or kill their target. 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. 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.

Left 4 dead boomer