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 maps. In the first two to four maps of any campaign, the survivors attempt to reach a safehouse, while the final stage 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 a rescue vehicle, or collect and deploy fuel cans to facilitate an escape.
Each survivor can carry one main weapon,and either a melee weapon or pistols as a secondary weapon. Though melee weapons cause extra damage to the infected when struck, the survivors can use any other weapon or item for weaker melee attacks and to temporarily push the infected back. Players also carry a flashlight which has infinite battery and makes it possible to see in the dark, but also causes infected to notice the survivors much faster. They may also carry a single first aid kit, special ammo pack, or a defibrillator; in addition to either pain pills or adrenaline shot. 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 Boomer bile, which can be thrown at infected to cause them to turn on each other.
Because many of the special infected can quickly finish off a survivor if they are not rescued, the game encourages players to stay and work together to traverse the level safely. To enable situational awareness of other players, players are shown the health and status of their fellow survivors. If a player does not have direct sight of another survivor, they will be shown the survivor's highlighted silhouette. As they take more damage from the infected, players move slower. If a survivor's health drops to zero, they become incapacitated and can only fight off the infected using pistols even if they are carrying a melee weapon until rescued by another survivor. If a character dies, they remain dead until the next level, unless revived by a defibrillator, or, in Campaign or Single-player mode, reappear in a "rescue closet" to be freed by the other survivors. Should all the survivors be killed or incapacitated, or if all the human-controlled survivors die, the game is over, and players must restart that chapter.
Left 4 Dead 2 retains the three game modes of the original game Campaign, Versus, and Survival and adds a new game mode called Scavenge. In Campaign mode up to four human players fight against the computer-controlled infected to make their way between safe houses; any survivor not controlled by a human player is controlled by the computer. In Versus mode up to four other human players take control of the various Special Infected who try to prevent the Survivors from reaching the next safe house. Special Infected are randomly assigned and one player typically controls a "non-pinning" infected which cannot "trap" a survivor (such as a Boomer or Spitter). Occasionally, as determined by the "AI Director", players will become the Tank, who has a massive amount of health and strength. The two teams swap sides once per chapter and are scored based on their play through as Survivors. Points are given during chapter progress, the more distance, the more points. If both teams make it to the saferoom with all four survivors, a 25 point tie-breaker is awarded to the team that dealt the most combined damage as special infected. Versus mode typically requires at least 1 human player on each team. Survival mode is 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. In the new four-on-four Scavenge mode, the survivor players are required to collect and use as many of the sixteen gas canisters scattered about a level to maintain the fuel of power generators, while the infected players attempt to stop them.
The game also features a realism mode, which can be enabled at any difficulty for either campaign or versus modes. This mode removes some of the video-game aspects from the gameplay: survivors are not able to see the silhouettes of their teammates; if they die, they can only be revived with a defibrillator kit and will not respawn later in the level. Damage dealt to Infected is also changed, with headshots dealing more damage to enemies, whereas limb or body shots require more shots, making gameplay even more of a challenge. Additionally, the Witch will now kill any survivor it would have merely incapacitated. Items such as defibrillators, throwable items and weapons will only glow when the player is within a few feet, forcing the players to investigate more thoroughly. Designed to force players to work closely together and rely on voice communication, the realism mode was created to give players a way "to be challenged as a team" without having to increase the difficulty level of the game.
