Set during the aftermath of an apocalyptic pandemic, the game focuses on four new Survivors, fighting against hordes of zombies known as the Infected, who develop severe psychosis and act extremely aggressive.

[4] Prior to release, it received mixed critical and community reactions, and attracted an unusually high volume of controversy about its graphic content.

[17] 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).

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.

The five Special Infected from the first game return in Left 4 Dead 2, some with modified behavior and skin models: Left 4 Dead 2 also introduces three new Special Infected, voiced by Dee Bradley Baker: As in the first game, an artificial intelligence system called the AI Director drives gameplay by procedurally spawning enemies, weapons and items based on players' performance during any given campaign.

[3] The Director now rewards players for taking longer or more difficult paths through each episode by providing useful equipment, such as special ammo packs or higher tier weapons.

[4] Like its predecessor, Left 4 Dead 2 is set in the aftermath of a worldwide pandemic of a disease nicknamed the "Green Flu", which rapidly transforms humans into zombie-like creatures and mutated forms that demonstrate extreme aggression towards non-Infected beings.

Left 4 Dead 2 introduces four new survivors: Coach, Ellis, Nick, and Rochelle, who are immune to the disease and have individual backstories that are provided through character dialogue.

At the end of the first game, the original survivors—Bill, Francis, Louis, and Zoey—arrive at the portside town of Rayford in Georgia in order to search for a boat that can take them to the Florida Keys.

Ellis helps the group use a stock car to break out of the mall[9] and travel towards New Orleans, rumored to be the last standing city in America.

After fighting their way through a wedding reception (complete with a Witch bride),[55] the streets, and a historic under-the-river tour,[56] they meet up again with the original survivors, who help by covering for the group while they fill the generator and lowering the bridge once it's full.

[10] At the start of the Dark Carnival campaign, the four survivors find the highway ahead completely blocked by ditched vehicles, and are forced to travel on foot through an abandoned (but still operating) amusement park.

[38] After navigating their way to the park stadium, Coach devises a plan to use a large-scale light show on the concert stage, which had been abandoned by a rock band called The Midnight Riders, in order to signal a helicopter pilot for rescue.

When the pilot starts attacking them, Nick is forced to kill him (echoing a similar situation in Left 4 Dead),[58] causing the chopper to crash into a bayou, which acts as the setting for Swamp Fever.

[60][61] As a tumultuous hurricane approaches (the titular Hard Rain of the subsequent campaign), the survivors go ashore at Ducatel in Mississippi, after which they make their way through an abandoned (and Witch-infested) sugarcane mill to a gas station to get diesel fuel,[62] and return to signal Virgil using the neon sign of a burger chain restaurant.

[76] Randall Newsome, the voice of the boat captain Virgil, was a local actor from Louisiana who naturally spoke Cajun and helped to embellish the role better than the writers could.

[74] One discarded idea for a new Infected included the "Leaker"; the creature, when having taken damage, would shoot out spouts of goo at the survivors, and then would be able to self-detonate like the Boomer.

[74] In development, the programmers wanted to find a way to provide more variation and visual effects for the players, although they were already strained by memory limitations on the Xbox 360 for Left 4 Dead.

One key development factor was recognizing a way to create greater variation in the appearance of the common Infected through a number of modeling and rendering tasks.

Simple changes of the underlying geometry of the model, either the head or the body, with the resulting effects on the texture mapping was one means to create apparent variation.

This prequel ties up the story of the original game, showing the events leading to the meeting of the two groups of survivors in "The Passing" from the point of the Left 4 Dead characters.

The film's director, Drew Goddard, commented on what the downloadable content would have included, stating: "The game was gonna be amazing.

[155][156][157] Confogl is a set of rules for competitive Left 4 Dead 2 Versus mode which standardizes certain elements and changes the distribution of items and weapons.

John Walker of UK-based gaming website Rock Paper Shotgun has theorized that this is due to higher expectations for Valve, given the perception of its status as an industry leader.

[167] This event, through a series of correspondences made in jest, shortly led to Valve's Gabe Newell and designer Erik Johnson flying to Australia to visit "Joe W-A", a Left 4 Dead modder.

It was deemed too explicit, and to appease the ESRB, Valve changed the image so that the fingers were merely bent back instead, but the company was still able to include the torn-off thumb in all regions except for Japan and Germany, where it had to be censored.

[182] However, as the appeal process, expected to end on October 22, was close to the planned release date, Valve submitted a modified version of the game for classification addressing the concerns the OFLC has stated.

[188] The Australian Federal Parliament passed legislation allowing an R18+ classification for video games on January 1, 2013,[189] and Valve stated they would explore legal options of resubmitting the uncensored version of Left 4 Dead 2 following this change.

[191] In order to achieve the highest possible rating given by Germany's Unterhaltungssoftware Selbstkontrolle (Freigegeben ab 18 Jahren gemäß § 14 JuSchG, meaning "Restricted for those below the age of 18"), Valve had to heavily censor the game's violent content similar to the Australian version.

However, the international (and thus uncensored) version was indexed by the Federal Department for Media Harmful to Young Persons on December 1, 2009 in order to prevent sellers from advertising or selling it to minors.