Select controller:Survivors
General survival
Above all, sticking together is crucial to survival in Left 4 Dead. If you get left behind or run on too far ahead you are easy prey for a special Infected. Generally speaking, there should be one player slightly ahead of the others to check out dangers up ahead, although close enough that the other players can provide assistance. Be especially careful when going around a blind corner or out from under an overhanging area: Boomers will often show up right where you can't see them (while you can listen for the belching noises you won't always hear him in time to avoid his vomit), and Hunters like to pounce on their prey from above if they can.
In Left 4 Dead, survivors are generally vulnerable due to their limited access to health, and having to proceed with limited ammunition for their primary weapons. As such, survivors have a best chance of survival trying to reach the safe house by following main passageways (or side passages that don't deviate away), with limited focus on collecting any scattered supplies.
Healing
The attacks of the Infected will deal damage, as will friendly fire from the other survivors. Health does not regenerate in Left 4 Dead, so wounds must be treated with a Medkit. At the start of a campaign a Medkit spawns on the table for each of the survivors, and each player should pick one up. Unfortunately, if you are playing with one or more AI survivors you may find it difficult to convince them to pick it up. You can heal yourself with the Medkit by selecting it and holding 
, or by getting close to another player and holding 
. Applying a Medkit takes some time but will heal most damage and won't disappear gradually like pain pills. Once successfully applied, the Medkit is used up. To get the maximum amount of health back from a Medkit you should save it until a survivor's health is down to red and then heal him with it.
In addition to Medkits, the Director will randomly spawn Pain Pills at certain points on the map. Unlike Medkits the Pain Pills work immediately, but only give a temporary health boost (indicated by the health bar made up of dashes that gradually shrink, rather than being filled in); damage can only be permanently restored by a Medkit. Pain Pills are consumed by pressing , or can be passed to another player by getting close and pressing . Unlike Medkits, Pain Pills received from another player sit in the inventory until used. Once swallowed, the Pain Pills are used up.
As well as the random spawns there are also first aid cabinets on certain maps; these cabinet are always in the same locations, but their specific contents are determined by the Director. A cabinet can contain anywhere from one to four Medkits or bottles of Pain Pills.
You can only carry one Medkit and one bottle of Pain Pills at any one time. If you already have all you can carry and find additional healing items you should get another player to pick up to ensure they are available for the group to use.
Infected
The infected Have quite a large diversity ranging form the common infected to large infected like the tank. The major infected include the common infected, the boomer, the smoker, the hunter, the tank and the witch.
The common infected
These are infected inhabitants of the place. They are infinite in number. The only thing they do to you is only clawing and scratching so they are not much of a threat to you if they are small in number. However, large groups(hordes) of these infected can inflict a considerable amount of damage and even constrict you. Plus, they act as a perfect cover for the special infected to attack making you group to disperse and making each survivor venerable for maximum damge infliction from the special infected.
The boomer
The boomer is a large, ugly and easy to kill monstrous infected in left 4 dead. The basic ability of a boomer is that it can vomit a greenish liquid onto a survivor or a group of them. This vomit attracts a horde of zombies to the survivor and lets causing them to inflict a considerable amount of damage. Once a survivor is covered by the vomit of a boomer, the survivor will be surrounded by a horde of zombies and he could die unless another survivor fights the horde and kill the horde. The boomer, unlike the other special infected, can be killed with one or two shots from the pistol or a single bullet fro the the other weapons. Although its that easy to kill a boomer, If you shoot the boomer while it is very close to you, he will explode onto you and the vomit will cover you and the same thing follows again. the best way to kill a boomer is to shoot it from distance or it its close to you, to shove it away then shoot it.
The smoker
The smoker is a thin, tall guy with a very long tongue and smokes around him. What the smoker basically does it it shoots its tongue towards a survivor wrapping him in his tongue and pulling him away from the rest o f the crew. And since the player is constricted he cannot fire his weapon. Thus, unless other survivors help out, the survivor will die. The smoker is a tough one so to kill him the best way is to land multiple shots from a shotgun or a SMG. smokers are usually on high places like buildings and cars so keep a watch out.
The hunter
The hunter is a very fast moving, high jumping, and creepy creature. The basic ability of a hunter is to jump from high places onto a survivor and pin them on the ground, inflicting massive damage. Once pinned, there is no way to fight against hunter. The only thing you can do is hope that someone else comes to your rescue. The best thing to do is to spot these monsters before they spot you.
One way to spot them is to listen for their distinctive scream. You also may be able to spot it as it jumps around. If you see anything jumping, shoot it -- it's a hunter!