The common infected are normal zombies with no special abilities like the special infected.
These are them just normal people turned into zombies and mutated by the green flu aka virus that made everyone infected this way.
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. Like cinematic zombies, those present in the Left 4 Dead series attack human victims until they themselves perish or kill their target.
In the cases of Common and Uncommon Infected, observed aggressiveness manifests itself primarily in the form of frenzied physical blows and kicks. Bite attacks are of a secondary nature and may even be incidentally delivered. 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). Common Infected are also frequently observed displaying mutual aggressiveness towards one another. However, these attacks cease when the Infected find and focus on an uninfected human.
Idk what else to put but we're moving onto the special infected now