The special infected are more powerful and stronger then common infected and have these abilities when mutated that help them kill their targets making them more dangerous and deadly.
The Special Infected are the mutated variants of Common Infected. Through some unknown mechanism of the Green Flu, a small percentage of Infected humans will undergo a dramatic mutation, eventually turning into one of these Special Infected, each with their own unique inhuman abilities. They don't spawn as regularly as the Common Infected do, but their abilities can easily wipe out a team of Survivors if not dealt with quickly, making them the true teamwork testers of the Left 4 Dead games.
The Special Infected have higher intelligence than the Common Infected. Special Infected are not affected by the beeping sounds of the Pipe Bomb. They will sometimes bide their time, waiting for Survivors to reach "ambush" areas before attacking them. Several Special Infected, like the Boomer, Spitter and Smoker may also hide after attacking/attempting to attack a Survivor.
Now I'll do the special infected one by one (this is gonna take forever 😭)