Left 4 Dead

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, the remaining survivors are filled by AI-controlled bots. They play through campaigns fighting off the infected—living humans who have been infected with a rabies-like virus that causes psychosis. The overarching goal is not to clear infected, but only to reach the next safe area alive. 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.

While a shooter, the game focuses largely on cooperation and teamwork, and thus eschews some "realism" conventions usual in other FPS games. Colored outlines of teammates are visible through walls to help players stick together and coordinate their movement. If a survivor's health is depleted, they become incapacitated and can only be helped by another survivor, after which they continue playing with lower health. If they are incapacitated twice without healing, the third incapacitation will kill them.

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. Memorial walls—giving names and obituaries to those who have died in the infection—suggest that the game takes place in October 2009. Two weeks after the first infection, four immune survivors—Green Beret and Vietnam veteran William "Bill" Overbeck (voiced by Jim French), college student Zoey (voiced by Jen Taylor), district account manager Louis (voiced by Earl Alexander), and outlaw biker Francis (voiced by Vince Valenzuela)—make their way out of the city of Fairfield, only to discover that the infection is creating dangerous mutations in some of its hosts.

After narrowly avoiding the new types of infected, the survivors are alerted by a passing helicopter to an evacuation point at the roof of Mercy Hospital. 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. Zoey is forced to kill him, causing the helicopter to crash in an industrial district outside the city. Finding an armored delivery truck, the group uses it to reach the small town of Riverside.

The concept of Left 4 Dead had its origins in a modification created by Turtle Rock Studios for Counter-Strike: Condition Zero, which they were co-developing with Valve. 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. Turtle Rock also were co-developers of Counter-Strike: Source, in which they refined the mod, refining the assets to make it more horror themed and having the players fight off zombies. Turtle Rock had attempted to get Valve to publish "Terror Strike" as its own game, but did not gain any traction until Valve writer Chet Faliszek discovered the project and championed it to Valve's CEO, Gabe Newell. Both versions of "Terror Strike" for Condition Zero and Source eventually were released to the Internet in 2023.

With Valve's approval, development on Left 4 Dead started in mid-2005, and grew significantly larger after Faliszek became lead of the project from Valve's side, bringing in over 100 Valve employees to help expand production. Turtle Rock aimed to create a horror film-inspired game that merges single player games' character-driven narrative structure with multiplayer games' social interaction and high replayability.

Left 4 Dead received highly positive reviews from critics. At Metacritic, the game received an aggregated score of 89 out of 100 for both Xbox 360 and PC. IGN stated, "It's almost pitch perfect in how it captures the tension and the action of a Hollywood zombie movie", and went on to describe it as, "quite possibly the perfect co-op shooter." Giant Bomb commented that the Source engine was beginning to show its age, but praised the game's use of lighting and filmic effects that gives the game world, "a desolate, washed-out feeling", as well as the realistic and emotive faces and the engrossing art direction.

Eurogamer concluded that Left 4 Dead "is another deeply professional, personality-filled and progressive take on the shooter from Valve." com praised the game's replayability, but GameSpot criticized the "limited map selection" that could "sometimes feel a bit repetitious". GameSpy noted the lack of an overall narrative between the campaigns was disappointing. However, some reviewers praised its faithfulness to the zombie film genre, including the "deliberately ambiguous" back-story, and the amount of characterization and emotion brought by each of the four survivors.