Left 4 Dead begins in Pennsylvania, following an outbreak of so-called "Green Flu", a highly contagious virus causing extreme aggression and loss of higher brain functions.
Two weeks after the infection of Patient Zero, four immune survivors of the outbreak (Francis, Zoey, Bill, and Louis) 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. ("No Mercy") With Zoey forced to kill the pilot, the helicopter crash lands in an industrial district outside the city. Finding a delivery truck that has been armored by other survivors, the group decides to use it to make their way to the town of Riverside ("Crash Course"). Before arriving in the town however, they find the road blocked, and are forced to travel the rest of the way on foot. Following an encounter with an infected madman in the local church, they discover that the town is overrun, and decide to head to a nearby boathouse for possible rescue. Contacting a small fishing vessel, they manage to reach the city of Newburg on the other side of the river, only to find much of it in flames ("Death Toll").
Seeking cover in a large greenhouse, their rest is interrupted when a military C-130 Hercules passes overhead, leading the survivors to travel through the city's business district towards Metro International Airport. Upon arrival, the group is shocked to see that in an attempt to contain the infection, the military had bombed the airport; the runway however is largely intact, allowing the survivors to fuel up and escape in the waiting C-130 ("Dead Air"). Despite this apparent rescue, a second crash occurs, and the survivors once again find themselves alone on the outskirts of Allegheny National Forest. Following a series of train tracks through the area, the group find themselves at a functioning, but abandoned, military outpost. After answering a radio transmission, the survivors make their final stand against hordes of infected, before a military APC arrives to transport them to Northeast Safe Zone, supposedly the only location in the area yet to be overrun ("Blood Harvest"). They are temporarily held by the military before the base is overrun with infected. The four escape via train and travel south at Bill's insistence; Bill believes that they can find long-term safety from the infected on the islands of the Florida Keys. (The Sacrifice comic) At the portside town of Rayford, they find a boat but must raise an old rusty bridge powered by an aging generator to get the boat into open waters, assured that the machinery noise will alert a large horde. When Louis is injured and unable to move quickly, Bill sacrifices himself against the horde to make sure the generator keeps running and allowing the three to stay clear of the horde.("The Sacrifice") After waiting for the horde to disperse, the three are taking time to mourn Bill's death when they encounter the four survivors from Left 4 Dead 2, who need to cross the bridge to continue to the west. ("The Passing") Zoey, Louis, and Francis help the four to get across and fend off another horde as they lower the bridge, and then set sail for the Keys.
Ellen Page (Juno,X men The Last Stand) as Zoey, a university student
Boris Kodjoe (Undercovers) as Louis, a district Account Manage
jason-statham (Transporter 1,2,3) Francis, a stereotypical biker
Liam Neeson (Taken,A-Team)as Bill Overbeck, a Vietnam Veteran
The infected (voiced by experimental musician Mike Patton and voice actor Fred Tatasciore)[24] are the survivors' foes in Left 4 Dead, and they appear to be partly inspired by the infected from several modern films, including Dawn of the Dead, 28 Days Later, or Quarantine.[25][26] An important distinction from the former is that while the infected do resemble traditional undead zombies, they are, as cited in the game's manual, living humans infected with a rabies-like pathogen, very much like the infected in the latter film. The manner in which they are portrayed is meant to create and sustain a more brutal, believable reality for the player to become immersed in. While they are never seen eating human flesh or brains, multiple 'survivors' who have been killed by the infected appear throughout the game, and appear to have been partially eaten. In The Sacrifice comic, Zoey's mother is bitten in the face. She then begins to attack the rest of her family. One such victim can be heard saying "I can't believe he bit me!" before being encountered later on, fully infected. In an interview with 1Up.com, Mike Booth commented on the concept of using a pathogen as an inspiration for the setting:
“ Even though we obviously pushed well beyond the realm of believability with many of our "boss" infected, the core idea of a mind-destroying, civilization-collapsing pathogen is more horrifying to me than magically animated corpses, precisely because it is plausible. Rabies is a good example of a pathogen that can turn a loyal, friendly, protective family pet into a slavering attack machine. It's a virus that reprograms the behaviors of a complex animal – a mammal, in fact. What if something similar happened to humans? Left 4 Dead is one possible answer.[6]
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The common infected encountered during the game are fast and agile, weak individually, but may be overwhelming in numbers. They display a special attraction to high-pitched alarm-type sounds, such as the beeping device attached to pipe bombs and car alarms. Common infected will often chase down the source of these noises while ignoring lower-pitched, but much louder sounds such as gunfire. They occasionally attack in masses referred to in-game as a "horde".[27]
In addition to the common infected, there are five "special" or "boss" infected whose mutations grant them special attacks that make them far more dangerous. Each of the special infected, as well as approaching hordes, have a distinctive sound or a timely musical cue, making their presence easily recognizable by players. There are five special infected in Left 4 Dead.[28]
The Boomer is a bloated infected that can spit vision-impairing bile on the Survivors that attract hordes of infected, and explodes on death, spewing more bile in the immediate area.
The Hunter is an agile foe that can jump and pounce on a Survivor, and then tear at his victim until he is knocked off or killed.
The Smoker has a long tongue that he can use to ensnare a Survivor and then drag him back to himself, either attacking the Survivor directly, allowing the horde to attack the Survivor, or leaving the Survivor suspended over a cliff or roof edge; the Smoker only releases its victim if the tongue is shot or on death, at which point it dies in a burst of vision-clouding smoke for a brief period.
The Tank is a gigantic muscular infected that has the ability to swipe at Survivors, knocking them off their feet and incapacitating them; it can also knock cars and other debris into the Survivors.
The Witch is a passive female infected that stays in a spot, crying; Survivors can sneak by the Witch, but if they should disturb her with light or loud noises, she will become aggressive and attack the Survivor that alerted her, usually incapacitating (unless on expert difficulty, thus having the Survivor die in one hit.) the Survivor and attacking him until she or the Survivor is killed.
Each of these special infected is preluded to players through special audio cues; for example, the Witch's cries or the Smoker's coughs can be heard before they are seen, while certain musical cues will alert the players to the presence of a Hunter in the area.
One special infected did not make it into the game. The "Screamer" was an infected strapped in a straitjacket and let out a loud, high-pitched scream when it sighted the survivors, thus forcing the survivors to find and kill it before it screamed, resulting in a horde of common infected. However, it ended up being too difficult for players to figure out where the Screamer came from or where it was going or even understand that they were meant to chase it. Because of the confusion, it was replaced by the Boomer.
In versus mode, four additional players can take control of the special infected—apart from the witch, who remains computer-controlled. Each infected player is randomly assigned a class of special infected when they enter a spawn mode. While in spawn mode, the infected can quickly roam around the map in search of an appropriate place in which to spawn. This location must be sufficiently distant from any survivor, out of the line of sight of any survivor, and outside restricted areas such as safe rooms. Upon death, the infected player must wait up to 10–25 seconds before reentering spawn mode, depending on how many players are on the infected team. When a tank is spawned in the game, infected players receive a message indicating which player will control it. The human-controlled infected can see their teammates' outlines through walls similar to the survivors, but can also see each survivor's outline, which is colored according to the survivor's health and fades out if the survivor refrains from attacking, running and vocalizing. Vertical pathways exclusive to the infected, such as pipes and vines, are marked with animated symbols for the infected players. These can be climbed and used for ambushes.