Set during the aftermath of a zombie outbreak on the East Coast of the United States, the game pits its four protagonists, dubbed the "Survivors", against hordes of the infected.

In all modes, an artificial intelligence dubbed the "Director" controls level pacing and item placements in an attempt to create a dynamic experience and increase replay value.

Left 4 Dead received praise for its replay value, focus on cooperative play, and cinematic feel, although some criticized its limited level selection and lack of narrative.

This is compounded by the presence of an "AI Director", who tracks each player's current situation, and will add or remove items and infected in response; with the goal of maintaining constant tension and creating a new experience with each playthrough.

(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.

)[7] All original campaigns are divided into five levels, connected by safehouses; checkpoints where players can heal, re-arm and revive characters who were killed.

[23] An outbreak of a highly contagious pathogen nicknamed the "Green Flu" (which causes extreme aggression, mutation to the body cells, loss of higher brain functions, and essential zombification of those who catch it) begins in Pennsylvania.

Fighting their way through the city's streets, subway, and sewers, they are rescued from the hospital's roof by the pilot (voiced by Dennis Bateman), only to discover that he is infected.

After encountering a paranoid and delirious man (voiced by Nathan Vetterlein) in the local church, they discover that the town is overrun and head to a boathouse for rescue.

Contacting a small fishing vessel (owner voiced by John Patrick Lowrie), they are kicked out of the boat at the city of Newburg across the river, finding much of it in flames.

The U.S. military had bombed the airport in an attempt to contain the infection, though the runway is partially intact, allowing the survivors to fuel up and escape in the waiting C-130 (pilot voiced by Gary Schwartz).

The survivors escape via train and travel south at the insistence of Bill, who believes they can find long-term safety on the islands of the Florida Keys.

This mod, named "Terror Strike", saw four players assigned as the terrorists with the goal to plant a bomb and then defend it from unending waves of counter-terrorists.

[7] To give Left 4 Dead significant exposure, Valve financed a $10-million marketing campaign for the game in the United States and Europe, with advertisements appearing on television, print, websites and outdoor placements in many cities.

Rendering and artificial intelligence were scaled up to allow for a greater number of enemies who can navigate the world in better ways, such as climbing, jumping, or breaking obstacles.

[7] Left 4 Dead underwent many phases of development; influenced by playtesting, Turtle Rock Studios removed many of the features that were originally in the game.

This would provide a score to rank a player's effectiveness as a teammate,[38] but this system was removed late in the development of the game in favor of immediate, non-persistent feedback displayed in-game.

Another significant element removed was a long introduction between campaigns; because the game is designed for replayability, it was difficult to hold the player's attention for repeated viewings of cut scenes, so they were dropped in favor of a sparse narrative.

[42][47] The game was first revealed in the Christmas 2006 publication of PC Gamer UK with a six-page article describing a playthrough at Turtle Rock Studios headquarters.

Turtle Rock Studios announced Left 4 Dead on November 20, 2006,[48] and was acquired by Valve on January 10, 2008, because of the game and long-standing relationship between the companies.

Valve later detailed in their official Left 4 Dead blog how they designed the movie, from a basic animatic in July 2008 to the final result for the launch of the game.

[61] A spokesman from Valve said, "The fix is designed to halt the cheating behavior on the dedicated servers, which accounts for the majority of the co-op and versus modes of play.

This included a new set of plugins that allowed for users to import data from SketchUp, a free 3D modeling program, directly into the Hammer level editor for use in maps.

[77] The beta was concluded on June 25, 2009, with the full release of the Left 4 Dead authoring tools and corresponding server and matchmaking update to support custom maps.

[97] However, some reviewers praised its faithfulness to the zombie film genre,[104][105][106] including the "deliberately ambiguous" back-story,[107] and the amount of characterization and emotion brought by each of the four survivors.

[102] Hideo Kojima, creator of the Metal Gear series declared in an interview with 1UP.com that he was "addicted to the game", which was, in his view, one of the "core titles made with movie-industry people that explore the depths of hi-def".

[112] On February 3, Electronic Arts revealed that Left 4 Dead had sold 1.8 million copies, excluding Steam and worldwide sales figures.

Left 4 Dead and its sequel created a format for several games that followed that used the same four-player-versus-environment style of gameplay using artificial intelligence similar to the AI Director and encourages strong cooperation between players.

[130] Seven of the original Left 4 Dead developers from Turtle Rock Studios were part of the team that released a spiritual successor titled Back 4 Blood in October 2021.

[135] Addressing concerns voiced by fans, Gabe Newell responded to an email from Kotaku explaining that despite the upcoming sequel, Left 4 Dead would continue to be supported and more content was planned in the coming months.