Cooldown

Cooldown is the minimum length of time in Left 4 Dead that a Survivor or an Infected needs to wait after using an ability before they can use it again.

The Survivors
''Main article: Melee

The Survivors have no special abilities and therefore does not need to wait to use their abilities other than healing, shooting and reloading, but there is an exception in Versus mode called Melee Fatigue. After couple of melee uses, the Survivors will need to wait 2 seconds until they can use it again, and if they will keep using it, they will have to keep waiting 2 seconds each melee attack until fully rested for couple of seconds.

Originally this was not in the game until Valve received complaints of people not actually "Firing their gun" in Versus and Campaign matches. When the Left 4 Dead DLC known as the Survival pack came out, Valve added in this new system but left it out from the Campaign mode for unknown reasons.

The Infected
The Infected have different abilities, and therefore different cooldown times.
 * The Boomer has to wait 30 seconds before being able to vomit on the Survivors again.
 * The Hunter does not have a cooldown on Pouncing, but it must wait 2 seconds before being able to leap if it stood up.
 * The Smoker's tongue takes 15 seconds to rewind if it breaks or is removed from a Survivor. In Versus, it takes 5 seconds to rewind if it misses.
 * The Tank takes 3 seconds to swing his arm again after punching and to rip the ground to throw concrete. The Tank has no punch delay after a rock throw, meaning it can hit a Survivor twice.

The Hunter, Boomer and Smoker's secondary claw attack has a 1 second cooldown.