The popularity of the game led to the development of a sequel, Left 4 Dead 2, which was released on November 17, 2009. A new map for both Left 4 Dead and Left 4 Dead 2, called "The Sacrifice", was released on October 5, 2010. In July 2012, all Left 4 Dead campaigns were ported over to Left 4 Dead 2, with cross-platform multiplayer support between Windows and Mac versions of the game.
Plot
Pennsylvania suffers an outbreak of the "Green Flu", a highly contagious pathogen that causes extreme aggression, mutation to the body cells, and loss of higher brain functions (essentially zombifying those who catch the flu). Two weeks after the first infection four immune survivors—William "Bill" Overbeck (voiced by Jim French), a Green Beret and Vietnam veteran; Zoey (voiced by Jen Taylor), a college student; Louis (voiced by Earl Alexander), a district account manager, and Francis (voiced by Vince Valenzuela), an outlaw biker—make their way through the city of Fairfield, only to discover that the infection is creating more dangerous mutations in some of its hosts. After narrowly avoiding these new infected, along with hordes of others (as seen in the introduction video), the survivors are alerted to the presence of an evacuation point at the nearby Mercy Hospital roof by a passing helicopter. Fighting their way through the city's streets, subway and sewers, they are rescued from the hospital's roof by the pilot, only to discover he is infected.
With Zoey forced to kill him, the helicopter crash lands in an industrial district outside the city. Finding an armored delivery truck, the group uses it to make their way to the town of Riverside. However, they find the road blocked, and travel the rest of the way on foot. After encountering a hostile and delirious man (voiced by Nathan Vetterlein) in the local church, they discover that the town is overrun, and decide to head to a nearby boathouse for 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. 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 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 a waiting C-130. An airplane is seen trying to land, only to crash and explode, giving insight into how the outbreak affected air traffic.
Despite this apparent rescue, it crashes as well, 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 (supposedly) transport them to Northeast Safe Zone Echo, implied to be one of the few (if not the only) safe zones still standing. Instead, they are taken to a military installation and informed that even though they are immune, they still carry the infection. 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.
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. However, the generator gives out. Bill sacrifices himself in order to restart it, so that the others may reach safety. After waiting for the horde to disperse, the three then encounter four more survivors. They move the boat to the other side of the bridge and help them re-lower the bridge so they can cross in their car. Afterwards, Louis, Zoey, and Francis head back to the boat and set course to the Keys.
Gameplay
Left 4 Dead is a first-person shooter in which the player takes control of one of four survivors; if human players are not available, then the remaining survivors are AI-controlled bots. They play through the levels fighting off the infected—living humans who have been infected with a rabies-like virus that causes psychosis. The virus does not affect them but they can transmit it to others as carriers.
The game focuses on cooperation and teamwork and thus eschews some "realism" conventions usual in most FPS games of the wider genre. Colored outlines of teammates are visible through walls to help players stick together and coordinate their movement. If a survivor falls off a ledge, then they may automatically hang onto it and can only be helped up by another survivor. If a survivor's health is depleted, then they become incapacitated and can only be revived by another survivor, at which point they continue playing with a low amount of health that decreases over time. If a survivor has been incapacitated and revived twice without tending to their wounds, then they will experience distorted black-and-white vision, and the next incapacitation will kill the character. If a survivor takes enough damage while incapacitated, or is not eventually helped up by teammates, then the incapacitated character will die. During "Campaign" mode, if a survivor is killed, then they will respawn in a closet or other enclosed space after a period of time (except during key points in the scenario), but must be freed by another survivor to rejoin the team. Otherwise, the player must wait until the next level. The AI is incapable of accessing the "respawn closets", so if all human player survivors are killed or incapacitated, players will have to restart from the last checkpoint. Survivors can share first-aid kits and pain pills and help each other heal. Left 4 Dead has friendly fire (which causes no damage on the easy difficulty mode), increasing the need for caution around other survivors.
The survivors communicate by voice commands that are accessed by quick menus, and some may sound off automatically when performing actions such as reloading or spotting infected. Over 1,000 unique lines have been recorded for each survivor. Additional communication of player actions is conveyed through character lights. Also, weapon-mounted flashlights and muzzle flashes help the players in determining whether their companions are shooting, performing melee attacks, reloading or moving. Due to control issues and the likelihood of players using a headset, the Xbox 360 version of Left 4 Dead omits the quick phrases feature.
