Melee

Melee 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 and resulting animation). 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, Smokers, or Jockeys.

Damage
The shove does not cause real damage, yet still it can kill the Infected. It takes 1 to 7 blows to kill a Common Infected (the Riot infected are, of course, impervious to shoving as well as any frontal attacks), and 4 blows to kill a Special Infected, with the exception of Tanks, Witches (when not stumbled from her stupor) and Chargers, which are immune to the attack. In addition Common Infected can be killed with one hit from behind when they are unaware of the Survivors. Sometimes they may get stuck on objects and become immune to shoves. They may also not be affected if shoved as they climb onto an object due to a glitch.

Survivors are immune to the shoving, even though they flinch and get halted when hit. As no damage is done by the shove, any such action on Survivors will not count as friendly fire; however, primary attacks with Left 4 Dead 2's Melee weapons will, although the damage (aside from chainsaw) is severely reduced.

Witches can be shoved when they are set on fire by means of Molotov or gas can though is ill-advised, especially on Realism Versus, any difficulty that has Realism enabled, or Expert as you may end up being stumbled by the Witch by being in her way and she'll take a swing at you, incapacitating or killing you.

Rescuing other Survivors
When another Survivor is pinned by a Special Infected, the best and quickest way to help them out is to meleeshove against either the Survivor or the Special Infected itself, which will free them instantly.

Hunters
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 occasionally claw at you while stumbling.

Other uses of the shove is to melee them as they jump towards you, temporarily stunning them and cancelling their momentum, saving you from extra damage from a long-distance pounce on Versus mode. When the Hunter is too close to attack with primary weapons (such as right in the middle of your team), clubbing the Hunter to death instead of shooting is a viable, and necessary, tactic on higher difficulties due to increased damage with friendly fire.

Smokers
A Smoker's drag can be interrupted with a quick swipe of your shoving, rendering it harmless for another 15 seconds. This means an effective team can save a Survivor from taking any damage if they react quickly 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 at the tongue itself, or shoot at the Smoker to kill it. If you melee, you minimize the damage taken by the victim while the Smoker has a high chance to get away; however, the Smoker will have to wait for cooldown and can still sustain some damage if Survivors are fast enough. If you kill the Smoker, the victim will sustain more damage, and it will take longer to shoot the Smoker than a quick melee. Although that same Smoker won't be able to grab you in worse situations down the road, the AI director or the Infected player played as Smoker will be able to re-spawn another Special Infected again later. Shooting the tongue requires some precise aiming, which may not be possible when surrounded by the horde, but will be required in Versus as meleeing the tongue off Survivors is no longer effective.

You can shove 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. If you are being constricted by a smoker that is close to you, you may be able to shove the smoker and free yourself without necessitating help from your team.

Jockeys
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. It is possible to 'Dead Stop' (melee as they jump towards you) the Jockey in any mode.

Chargers
While shoving the Charger will do damage, it will not stun him in any way, so it is not recommended. If he charges and catches a Survivor, the shove it will not make it let go; requiring gunfire or one to two proper attacks from a melee weapon to kill it.

Melee Spam
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 or Jockeys. 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 re-spawn and set up an ambush. This is not a viable strategy in Left 4 Dead 2 due to the Chargers' ability to ram groups of survivors and Spitters' ability to create a field of acid under the survivors, causing significant damage to all of them.

Common times to see melee spam include:
 * 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. There is a HUD display that will show your melee fatigue. If a Survivor swings too many times in a short interval of time, then they become fatigued and the HUD display will show the cool down of the melee swing. This introduction to the game has helped deter melee spam and corner meleeing as the Survivors when a Horde attacks. It is applied only to the Versus and Survival modes of Left 4 Dead, but is present in all modes of Left 4 Dead 2.