The Infected (As you already know) are the primary enemies in Left 4 Dead Eleventy.
See Common Infected for more.
The Common infected are the basic and weakest infected,on Wimpish-Easy they die in one shot,but they can be troublesome on Expert-Freakin' Insane.
When the appear in a horde,using an automatic weapon like the Light Machine Gun or Heavy Attack Shotgun can save your life,or a well placed missile from the Missile Launcher or RPG Launcher can end the horde.
See The Deciever for more
The Deciever is one of the newer infected,with the ability to blow smoke onto the survivors to make them hallucinate and see things such as safety,ammo and tanks.