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Left 4 Dead 2 is a 2009 multiplayer survival horror game developed and published by Valve. The sequel to Turtle Rock Studios's Left 4 Dead, it was released for Windows and Xbox 360 in November 2009, Mac OS X in October 2010, and Linux in July 2013.

Left 4 Dead 2 builds upon cooperatively focused gameplay and Valve's proprietary Source engine, the same game engine used in the original Left 4 Dead. Set during the aftermath of an apocalyptic pandemic, Left 4 Dead 2 focuses on four new Survivors, fighting against hordes of zombies, known as the Infected, who develop severe psychosis and act extremely aggressive. The Survivors must fight their way through five campaigns, interspersed with safe houses that act as checkpoints, with the goal of escape at each campaign's finale. The gameplay is procedurally altered by the "AI Director 2.0", which monitors the players' performance and adjusts the scenario to provide a dynamic challenge. Other new features include new types of special infected and an arsenal of melee weapons.

The game made its world premiere at E3 2009 with a trailer during the Microsoft press event. Prior to release, it received a combination of positive and negative critical and community reactions. It attracted an unusually high volume of pre-release controversy about the game's graphic content. In response, alterations were made to the cover art and both Australia and Germany refused to rate the unmodified edition at the time of release. After release, however, the game was met with positive reviews by critics.

Gameplay
Like its predecessor, Left 4 Dead 2 is a first-person shooter with a heavy emphasis on cooperative gameplay. The game presents five new campaigns, each composed of three to five smaller levels. As in the first game, each campaign is presented in menus and loading screens as a film starring the four Survivors; and features, upon completing a campaign, a faux credits screen which acts as the final scoreboard for the players and recaps certain performance statistics for the group. In every campaign, most levels involve the Survivors attempting to reach a safe zone and close the door once they are all safely inside. However, the final level in each campaign requires the Survivors to call for rescue and either survive a prolonged onslaught until rescue arrives, pass through an especially challenging gauntlet of Infected to reach an escape vehicle, or (in Dead Center and The Passing) collect and utilize fuel cans to enable their escape.

Each Survivor can carry one each of five categories of equipment: primary weapons, secondary weapons, throwable weapons, major medical supplies (which also includes ammunition upgrades) and minor medical supplies. Primary weapons are broken down into four tiers based on their availability and attributes. Tier one weapons (such as the SMGand Chrome Shotgun) deal damage relatively slowly and serve as basic offensive weapons. Tier two weapons (such as the Assault Rifle and Combat Shotgun) deal damage faster and often appear in later maps of each chapter. The two tier three weapons (Grenade Launcher and M60) differ from tier one and two weapons both because of their rarity and because players cannot refill their ammunition from ammunition dumps. They deal damage quickly at the cost of limited ammo. Tier four weapons are all mounted weapon systems and carry unlimited ammunition. The mounted weapons deal damage the fastest, but at the cost of overheating after a period of non-stop use. The mounted weapons are in set positions in maps and have a fixed arc of fire. They cannot be given ammunition upgrades like other weapons and players cannot utilize the laser sights attachment with them. There are three types of secondary weapons available: melee weapons, small caliber pistols (that can be dual wielded) and large caliber pistols. Although melee weapons cause extra damage to Infected when struck, the Survivors can use any other weapon or item for weaker melee attacks that can push the Infected back. Players also carry a flashlight with infinite battery life, the use of which enables players to see in the dark (with the downside of the player gaining the attention of the Infected much faster). They may also carry a single first aid kit, special ammo pack, or a defibrillator in their major medical supply slot; in addition to either pain pills or an adrenaline shot in their minor medical supply slot. They may also carry a single throwable weapon—Molotov cocktails to set an area on fire; pipe bombs to attract any nearby Infected to the flashing light and sound it makes until it explodes; and jars of Boomer bile, which attracts Common Infected to whatever it hits (e.g. fires, Special Infected, each other).

To enable situational awareness of other Survivors, players are shown the health and status of their fellow survivors. If a player does not have direct sight of another Survivor (e.g. a wall is blocking their view of the others), they will be shown the Survivor's highlighted silhouette. The Survivors are also susceptible to friendly fire, forcing players to exercise caution when shooting or swinging their weapons. As Survivors take damage, they move more slowly; if a Survivor's health drops to zero, they are incapacitated and left to fight off the Infected using a handgun until rescued by another Survivor. If a Survivor dies, they remain dead until the next level, unless revived by a defibrillator, or, in Campaign or Singleplayer mode, reappear in a "Rescue Closet" to be freed by other Survivors. Should all Survivors die or be incapacitated, the game will end, at which point the players may restart that chapter or quit the game.

Game modes
Left 4 Dead 2 includes 5 regular game modes:


 * Campaign: up to four human players fight against the Infected to make their way through Campaign stages; any Survivor not controlled by a human player is controlled by the computer.
 * A Singleplayer mode allows players to play without intrusion from human players, working through the Campaign stages with three computer-controlled Survivor allies.
 * Versus: up to four other human players take control of various Special Infected to try and prevent the Survivors from completing a stage. Special Infected are randomly assigned to Infected players; they cannot control the Witch or any Common Infected. Occasionally, as determined by the AI Director, certain players will become the Tank. The two teams swap sides once per stage, and are scored based on their stage progress as Survivors. If both teams make it to the saferoom with all four Survivors, a 25-point tiebreaker is awarded to the team that dealt the most combined damage as Special Infected.
 * Survival: a timed challenge where the Survivors are trapped in a section of the campaign maps, and try to survive as long as possible against an unending onslaught of Infected.
 * Scavenge: a new 4-on-4 mode that requires the Survivor players to collect and use as many fuel cans scattered about a level to fill up a power generator, while the Infected players attempt to stop them.

Left 4 Dead 2 also features a Realism mode, which can be enabled at any difficulty for either Campaign or Versus. This mode removes some of the video-game aspects from the gameplay: Survivors cannot see each other's silhouettes, and dead teammates can only be revived with defibrillators and will not respawn later in the level. Weapons and other items will only glow when the player is within a few feet, forcing the players to search the levels more thoroughly. Headshots to enemies deal more damage, whereas limb or body shots require more shots. In addition, the Witch enemy can kill any Survivor she would have normally incapacitated (on difficulties other than Easy). Designed to force players to work closely together and rely on voice communication, Valve created Realism mode to give players a way "to be challenged as a team" without having to increase the difficulty level of the game.

Finally, in later versions of the game, there is a Mutation option, in which players can select from a menu of 30 variations, some singleplayer, some multiplayer (and some of the latter cooperative, some versus, or both). Each variation changes one or more major game dynamics, such as: constantly draining health, solo mode with no Survivor allies, death upon incapacitation, only one kind of weapon available, coop that later switches to versus, shooting or targeting constraints, an enemy type with increased power, increased numbers of Special Infected, etc. One of these modes, named GunBrain, is a weapons stats analyzer.

The Infected
While also referred to as zombies, the Infected are humans who have contracted a mutated strain of an infection, though neither the source nor nature of this "Green Flu" are made clear in the games. The most numerous Infected encountered by the survivors are the common Infected. Though individually weak, they can swarm and overwhelm the Survivors, especially when separated from their teammates. Damage to the infected in Left 4 Dead 2 is portrayed more realistically, with bullets and melee weapons ripping off bits of flesh and, in some cases, limbs. A new addition to Left 4 Dead 2 is the Uncommon Infected unique to each campaign. By virtue of location and equipment worn pre-infection, they possess abilities that separate them from the Common Infected. For example, the Dead Center campaign introduces infected CEDA agents in hazmat suits, making them fireproof; Dark Carnival includes clowns, whose squeaking shoes attract small hordes of Common Infected; and The Parish includes infected private security contractors in riot gear, making them bulletproof from the front.

As in the first game, there are special Infected whose mutations grant them special abilities that make them highly dangerous, which act as bosses. The presence of such Infected nearby is hinted at by sound effects or musical cues unique to each type. The five Special Infected from the first game return in Left 4 Dead 2, some with modified behavior and skin models:


 * The Boomer: an extremely bloated Infected who vomits bile. The bile attracts a horde of Common Infected on contact with Survivors and also momentarily blinds them. Upon death, they explode and can also spray bile onto nearby Survivors.
 * The Hunter: an agile male Infected that can pounce on and incapacitate Survivors from great distances, tearing at them until the Survivor dies or another Survivor shoves him off/kills him.
 * The Smoker: a male Infected with a long tongue that it uses to ensnare Survivors from a distance, continually choking them. Upon death, he releases a cloud of smoke that can obscure the Survivors' vision and cause them to cough, should they come in direct contact with the smoke.
 * The Tank: a gigantic, extremely muscular Infected male with the strength to punch Survivors off their feet some distance, as well as toss cars and concrete slabs. Unless the Survivors work as a team, they will be quickly incapacitated or even killed by the Tank's inhuman strength.
 * The Witch: a crying Infected woman who, when provoked by either damage, loud sounds, light, or proximity of survivors, will attack the provoker and any other Survivors after her provoker is dead. She can incapacitate or even kill a Survivor in one hit. Left 4 Dead 2 introduces a variant of the Witch that wanders aimlessly in the open.

Left 4 Dead 2 also introduces three new Special Infected, voiced by Dee Bradley Baker:


 * The Charger: a male Infected with an enormous right arm, and can charge into the Survivors, capturing and carrying one Survivor away from the others, sending any others in his path flying to the side after a capture and stunning survivors upon hitting an obstacle in a charge. He may then pummel his captured Survivor into the ground, rendering that Survivor helpless until one of their teammates helps them by killing the charger or stunning it with frag rounds.
 * The Spitter: a female Infected that spits out balls of stomach acid that splatter across an area, quickly eroding the Survivors' health as long as they remain within it. She also leaves a puddle of goo upon death. The longer a player loiters in the goo, the faster their health drops.
 * The Jockey: a male Infected that jumps onto a Survivor's back and steers them into other Infected or environmental hazards (e.g. Spitter goo), whilst clawing at the player's head until the player is incapacitated, killed, or shoved off.