Melee

Melee (pronounced MEY-ley) is a form of attack which shunts the target and renders them stunned for a small amount of time (1-3 seconds, depending on infected type). It is possible that a melee attack will knock Common Infected down. They must stand back up again before they can attack, giving you ample time to either kill them or run away. It is extremely useful in defense against large hordes of Common Infected and rescuing other Survivors caught by Hunters or Smokers.

Damage
The melee attack does not cause damage to the Common Infected nor the Special Infected. However, Common Infected can be killed with one well-aimed hit from behind, or four hits from the front. Sometimes they get stuck on objects and become immune to melee attacks. With the exception of The Tank, The Witch and The Charger when he is smashing a survivor, which are immune to melee attacks, it takes four melee attacks to kill a Special Infected regardless of the direction of the attack. Survivors are immune to the melee attack, even though they flinch when they are hit. Keep this in mind, as the success of the campaign might just depend on it.

Rescuing other Survivors
When another Survivor is pinned by a Hunter or snared by a Smoker, the best and quickest way to help them out is to use your melee attack against the Survivor or the Smoker or Hunter, which will free them instantly.

Hunter attacks
A Hunter will quickly tear away at a Survivor's health, so if you're low on ammo, or reloading, it can be best to use your melee attack against the Hunter to save them. Once a Hunter is knocked off, they will be temporarily stunned. When the stun wears off they will either attempt to make an escape to set up another ambush, or pounce immediately after. Be aware, especially on higher difficulties, that the hunter while stunned will occasionaly glitch and be able to melee while stumbling.

Smoker attacks
A Smoker's drag can be interrupted with a quick swipe of your melee attack, rendering it harmless for another 15 seconds. This means an effective team can save a Survivor from taking any damage if they react quick enough (though in Versus the Smoker will damage as it drags). When someone is first caught by a Smoker, his or her teammates must make a fast decision. Either melee the tongue off of the victim, or shoot to kill the Smoker. If you melee, you minimize the damage taken by the victim, but you give the Smoker a high chance to get away and smoke someone again, perhaps in a worse situation. If you kill the Smoker, the victim will sustain more damage, it will take longer to shoot the Smoker than a quick melee. This may help you in the long run, however, because then that same Smoker won't be able to grab you in worse situations down the road, such as on top of buildings or after a Boomer has blinded everyone. You can also use your melee attack against the Smoker to free the Survivor. A Smoker will be stunned for about 1 second after their tongue breaks, meaning you have some time to deal damage to it before it retreats.

Jockey attacks
Meleeing a Jockey off a Survivor can be difficult since the Jockey steers the victim away from you. If you do catch up to it do a simple melee on the Jockey (not the survivor) and it will fall and become temporarily stunned. Like the hunter it might try to jump again, scratch you, or run away.

Melee Spam
As of the latest update it is impossible to do an effective melee spam in versus mode.

Melee spam is a phrase used to describe the constant and prolonged use of the melee attack to prevent attacks from hordes of Common Infected and pounces from Hunters. Usually when a Survivor becomes a victim of a bile attack they will back up into a corner and continuously use their melee attack until the hordes of infected have died down. This is because they cannot see their surroundings or the auras of other Survivors, and so the general panic of a massive attack causes them to go totally defensive.

Corner meleeing (also corner stacking, corner spamming) is when one or more Survivors sit in a corner and continuously spam their melee attack, making an almost impenetrable defense against all types of attacks except a Tank. Attacks which can have an effect include a Hunter pouncing above the Survivors and landing on one of them, a well timed Smoker attack, and a Tank attack. A Boomer will only extend the amount of time they spend in the corner, giving the other infected on the team time to respawn and set up an ambush.

Common times to see melee spam include: The best ways to attack a survivor who does this is from behind,as charger,smoker or spitter. As hunter it is possible to pounce and land on top of them, especially if they are in a corner.
 * Crescendo Events,
 * Panic Events,
 * Boomer attacks.

Melee Fatigue
With an update for the PC and in the Survival Pack DLC for the Xbox 360, Valve has released a "melee fatigue" system. This system will help decrease melee spam in Left 4 Dead. There is now a HUD display that will show your melee fatigue. If you swing too much, then the Survivor you're playing as will become fatigued. The HUD display will show the cooldown of your melee swing. This introduction to the game has helped halt melee spam and bunching up in a corner as the Survivors when a Boomer vomits on them. It is applied only to the Versus and Survival modes, but not the Campaign or Single Player/offline Co-op modes. It is, however, in both of these modes in Left 4 Dead 2.