Plot

Left 4 Dead begins in Pennsylvania, following an outbreak of the so-called Green Flu, a highly transmittable STD-like virus which manifests itself as massively increased aggression and the loss of many higher brain functions within its victim.

Two weeks after the Infection of Green Flu, four immune Survivors of the outbreak make their way through the city of Fairfield, only to discover that the virus is creating new, even more dangerous mutations. After narrowly avoiding these new Infected, along with Hordes of others, the Survivors are alerted to the presence of an evacuation point at the nearby Mercy Hospital by a passing news helicopter. Fighting their way through the city's streets, subway and sewers, the Survivors are rescued from the hospital's roof by the pilot, only to discover that he is succumbing to the Infection.

First of all, when you do a campaign or other mods, you'll always have the choice between 4 characters.

Zoey: Zoey is one of the four playable survivors in L4D. She is 18 years old, the youngest of the team but also the smartest. She has a close relationship with Bill such as a daughter/father relation.
After spending her first semester holed up in a dorm room watching old horror movies, Zoey was given a choice: Stop fooling around and get her grades up, or drop out. Now that the planet's overrun with murderous zombies, and all of her professors are dead, Zoey at least has the cold comfort that she's been studying up on the right subject after all


William "Bill" Overbeck Bill is one of the four playable survivors in L4D. At 65 years old, he's the oldest of the team.
He is a Vietnam veteran who served in the US Army 1st Special Forces Group. He is usually seen wielding the Assault Rifle (which appears as the M16, a real life assault rifle used by the U.S. Military in Vietnam), probably to complement his back story. He often has a lit cigarette in his mouth.

Louis Louis is one of the four playable survivors in L4D. At 30 years old, he is the optimist of the team. He gets along with everybody. Louis had been working up the courage to quit his job as Junior Systems Analyst at his company's IT department when a virus showed up and downsized the world. Now Louis has a new set of goals (live long enough to succeed) and a new set of tools (guns, sharp objects) to help him achieve them. With any luck, he'll figure out how the new management operates before they get a chance to murder him

Francis Francis is one of the four playable survivors in L4D. At 38 years old, he's a biker. Of the four original Survivors, Francis is the most cocky and insulting.Francis is notable for often contextually exclaiming how he hates something. Why Francis hates almost everything is unknown, but it can be assumed that it has something to do with his past of being an outlaw, or perhaps just a personality quirk added for comic relief.

Left 4 Dead includes 6 campaigns. 4 original campaigns + 2 specials campaigns in the DLC.

No Mercy
The story starts with No Mercy, the first campaign. "Mercy" is the name of the hospital where the survivors need to go to evacuate the city. The campaign has 5 chapters/levels : The Apartments, The Subway, The Sewer, The Hospital and The Rooftop Finale.


Crash Course (DLC)
It's a special campaign included in the DLC.
Crash Course begins shortly after the events of No Mercy. The helicopter rescue pilot from the Rooftop Finale succumbs to the Infection and Zoey shoots him in mid-flight. The helicopter crashes in an industrial area outside of the city of Fairfield in Whitney County.
Crash Course is the only campaign in Left 4 Dead to start at the rescue vehicle of the previous campaign.


Death Toll
Spanning five chapters, Death Toll is set in and around a small (cf. road side population sign) town named Riverside where the Survivors believe the military may be holding out. The chapter takes place after the Crash Course campaign in which the survivors head to Riverside and their motivation for striving to reach the town center is based on no more than a vaguely expressed notion that the military may be holding out in Riverside, and a shared awareness that they have no other realistic option.


Dead Air
The campaign opens with the Survivors hiding in a rooftop greenhouse when a low-flying C-130 sparks agreement that the local airport may still be operating evacuation flights. Thereafter the campaign has The Survivors make their way through Newburg to Metro International Airport where they must refuel their escape aircraft.


Blood Harvest
After their rescue aircraft is lost, the Survivors hide out in a forest until learning of a nearby military outpost located at a barricaded farmhouse. They set off through the forest and follow a series of train tracks until entering a military controlled area only to discover the outpost is abandoned. A radio call to the army base at Millhaven triggers the finale and they must hold out until an armored personnel carrier arrives.


The Sacrifice (DLC) (Might contain spoilers)
Last campaign of the game, The campaign's name comes with the obvious interpretation that one of the Survivors has to sacrifice themselves in order to enable the survival of the team. Canonically, Bill is the one who makes this choice, as seen by his unfortunate appearance in The Passing's finale inside a generator room, as well as the comic. The tagline, "It's your funeral," refers to the idiom "it's your funeral", basically meaning someone may have made a bad choice, and if they follow up on it, they will have to endure the direct consequences regarding that choice. Should the survivors decide to sacrifice nobody, a continual horde and endless amount of tanks will spawn until they eventually overrun the survivors.

Left 4 Dead is all about killing zombies, but some of them are special. You have the common infected (normal zombies) and the special infected.

Common Infected : Common Infected, as their name implies, are the most commonly faced enemy and are the easiest to defeat. They are more akin to the antagonists from the film 28 Days Later than to the typical shambling cinematic zombie. They are exclusively AI-controlled and fairly weak. Their strength lies in their tendency to attack in large groups of up to several dozen, known as Hordes.

Special Infected : The Special Infected have higher intelligence than the Common Infected, as they are not attracted by car alarms or loud noise and are able to run away if required to set up an ambush. They are also more aggressive than the Common Infected, as they will actively attack Survivors rather than passively sitting in a stupor until disturbed.

Boomer
The Boomer is a Special Infected that vomits a special bile at Survivors, which temporarily blinds them and draws a Horde of Common Infected to them. At a distance the Boomer isn't much of a threat; their large size and low health make them an easy target against any weapon at mid or long range. However, if within range, they can vomit on Survivors, attracting the Horde.When they die, they explode, covering any Survivors who are close enough with bile, blinding them and causing them to stumble backwards.

Hunter
The Hunter is a Special Infected that has the least visible mutations, and can be mistaken for a Common Infected. The Hunter is extremely fast and agile, capable of quickly scaling walls. Additionally, he can pounce and pin down Survivors, clawing at their abdomen until either the victim is dead or the Hunter has been knocked off or killed by another Survivor. Hunters are not the most physically robust Infected, having relatively low health, and thus it doesn't take very much to kill them.

Smoker
The Smoker is an ambush class that can strangle Survivors with their long tongue. A Smoker also will frequently cough, letting the Survivors know when he is in the area. The Smoker can drag players that he has captured with his tongue towards himself, though he remains stationary and cannot move. You can see smoke when you kill him.

Tank
The Tank is the biggest and strongest of all the Infected and is reasonably fast, only slowing if hit with large amounts of automatic weapons fire or when lit on fire.The Tank has incredible strength and resistance―his punch can send Survivors flying, rendering them temporarily defenseless. It's more rare to see him than the others special but once he got you, he'll try to kill you.

Witch
The Witch generally only appears a few times per campaign, and is passive unless provoked by flashlights, gunfire, or Survivors in close proximity. She is the second "Boss Infected" (after the Tank) She is very powerful, capable of incapacitating and even killing Survivors with one hit. The Witch cries loudly (helps you to know where she is). When nearby, Survivors will mention they hear her and warn teammates to turn off their flashlight. If you don't want problem with her, you can just walk pass her, flashlight off.

Note: In Versus mod, all these infected are playable , except the Witch.

Here are the weapons you can find everywhere in the game.
Primary Weapons
-Pump Shotgun
-Submachine Gun
-Auto Shotgun
-Assault Rifle
-Hunting Rifle

Note: You can headshot a Witch with a Shot Gun if you are very close to her. You can also hurt a lot the Tank if you're close of him with your Shot Gun.

Sidearms
Other than primary weapons, Survivors are provided with a sidearm weapon at all times in case their primary runs low on ammo.
-M1911 Pistol

Note: You can carry 2 pistols

Grenades
Each of the Survivors can carry at any time one of the following grenades.
-Molotov cocktail
-Pipe bomb

Note: The Molotov is good against the Tank. The Pipe bomb is good against Horde of Zombies.

Usable Items
These items are found throughout campaigns, and when picked up, you can't use any weapon until you throw them or drop them. You can shove with them, however. When shot, burned, or otherwise detonated, these items blow up, spread flames, or both, making good tools for defending areas during Crescendos or Finales
-Gas Can
-Oxygen Tank
-Propane Tank
-Explosive Barrel

Fixed Weapons
Over the course of the campaigns, the Survivors find fixed gun emplacements left behind by the military in their failed attempt to contain the Infection. They appear at certain finales and crescendo events and have an infinite supply of ammo.
-Minigun
-Heavy Machine Gun (The Sacrifice)

Healing Items
Healing items are those used to restore lost health, be it a temporary health boost or a permanent restoration. They can be found in ambulances, safe rooms, or randomly throughout levels, but are most often found inside of first aid stations. First aid stations may contain up to four bottles of pain pills, but there is also a chance that they contain First aid kits instead, depending on how well the Survivors are doing and the difficulty level. First aid stations found in safe rooms and at finales always contain first aid kits.
-First Aid Kit
-Pain Pills

Once you finish all the campaigns in Left 4 Dead, there is a sequel of the game : Left 4 Dead 2.
New guns, new enemies, new campaigns and new characters.
Available now on Steam.

Left 4 Dead 2