Left 4 Dead

Left 4 Dead is a 2008 first-person shooter game developed by Valve South and published by Valve. It was originally released for Windows and Xbox 360 in November 2008 and for Mac OS X in October 2010, and is the first title in the Left 4 Dead series. 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.
Left 4 Dead uses Valve's proprietary Source engine, with four game modes: a single-player mode in which allied characters are controlled by AI, a four-player co-op campaign mode, an eight-player online versus mode, and a four-player survival mode. 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. Considered one of the greatest video games ever made, the game won several publication awards, as well as distinctions from the Academy of Interactive Arts & Sciences and British Academy of Film and Television Arts. As was done with Team Fortress 2, Valve supplemented the game with free downloadable content. The success of the game led to the development of the sequel Left 4 Dead 2, which was released in 2009. In 2012, all Left 4 Dead campaigns were ported over to Left 4 Dead 2, with cross-platform multiplayer support between Windows and Mac versions of the game.

Gameplay

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.
During "Campaign" mode, if a survivor is killed, they will eventually respawn in a closet or small room, but must be freed by another survivor to rejoin the team. The AI will not open "respawn closets", so if all human player survivors are incapacitated, the level will restart. Survivors can share first-aid kits and pain pills, and heal each other. Left 4 Dead has friendly fire, increasing the need for caution and coordination around other survivors.
Via a quick menu, the survivors can communicate and organize with voice commands and callouts. Over 1,000 unique lines were recorded for each survivor. Additional communication of player actions is conveyed through lights; weapon-mounted flashlights and muzzle flashes help players determine when their companions are shooting, performing melee attacks, reloading or moving.

Campaigns

All original campaigns are divided into five levels, connected by safehouses; checkpoints where players can heal, re-arm and revive characters who were killed. Some levels are broken up with "crescendo events", which introduce a new obstacle before the survivors can continue moving. In the final chapter of each campaign, the players must defend a position from an onslaught of infected until rescue arrives. The four campaigns are: "No Mercy", a city culminating in a hospital skyscraper; "Death Toll", a small town and countryside; "Dead Air", a bombed city and an airport; and "Blood Harvest", a woodland and farm setting.
A two-level DLC campaign, "Crash Course", was released on September 29, 2009, set in the industrial outskirts between "No Mercy" and "Death Toll". "The Sacrifice" is a three-level campaign released on October 5, 2010, with an industrial port-side setting in Georgia. These new levels contain a number of alternate routes to follow with more supplies, helping to create a sense of non-linearity. Each campaign typically lasts between 20 and 75 minutes depending on the difficulty level. Both platform versions of the game utilize an achievement system.

Weapons

Each player starts the game with a semiautomatic pistol, an M1911. It is the only weapon with unlimited ammo, and the only weapon that can be used while incapacitated. At the beginning of each campaign, survivors can choose between a submachine gun and a pump-action shotgun. As the survivors progress through a campaign, more powerful weapons can be found; the full-auto assault rifle, a semi-automatic shotgun, and a scoped hunting rifle.
In addition to firearms, a player can carry three other items: improvised grenades ; a first aid kit, which can be used to heal any survivor, and pain pills, which provide a temporary health boost. A melee attack can always be used to shove away any infected within reach. Also available are environmental weapons, such as gas cans, propane cylinders, and oxygen tanks, which explode when shot. These can be picked up and moved by survivors, but this blocks them from using other weapons at the same time. Rarely, there are strategically placed minigun turrets near crescendo events and finales.

Enemies

The Infected are the enemies of Left 4 Dead, and comparisons have been made with 'zombies' from certain modern films, such as Dawn of the Dead, 28 Days Later, and Quarantine. An important distinction from the former is that the infected are, as cited in the game's manual, living humans infected with a rabies-like pathogen; more similar to the latter two films, rather than undead zombies. While they are never seen eating humans, bite wounds and mutilation are often mentioned. In an interview with 1Up.com, designer Mike Booth commented on the concept of using a pathogen as an inspiration for the setting:
The "common infected" are fast and agile, weak individually, but overwhelming in numbers. They display a special attraction to high-pitched sounds, such as car alarms and the beeping device attached to pipe bombs. They will often chase down these noises while ignoring lower-pitched, louder sounds, such as gunfire. They occasionally attack en masse, referred to as a "horde".
There are five "special" or "boss" infected, whose mutations grant them special attacks. Each of the special infected have a distinctive sound, silhouette, and musical cue, making their presence easily recognizable by players. There are five special infected in Left 4 Dead:

The Boomer is a bloated infected that can spit vision-impairing bile on the Survivors. This bile attracts hordes of common infected. The Boomer explodes on death, spewing more bile; this came out of the desire to have an enemy players would not want to shoot on sight.
The Hunter is an agile enemy that can pounce a survivor from long distances, tearing at them until he is pushed off or killed.
The Smoker is an infected with a long tongue, which can be used to ensnare and drag a survivor back to himself. The survivor may be attacked directly, attacked by the horde while immobile, or left suspended in mid-air. The tongue is only released if a teammate shoves the ensnared survivor, shoots the tongue, or shoots the Smoker, resulting in a vision-obscuring cloud.
The Tank is a gigantic, muscular infected that has the ability to swipe at survivors, knocking them off their feet and incapacitating them; he can also throw rocks, cars, and other debris at the survivors. The Tank is the most durable of all the special infected, and requires the full support of all survivors to be killed. The Tank appears rarely, usually during crescendo events or finales.
The Witch is a female infected with long claws, who stays still in one spot. Survivors can often avoid the Witch, but if they disturb her with light, loud noises, gunfire, or getting too close, she will become aggressive and charge. The Witch is the strongest infected in the game, and on the hardest difficulties, will kill a survivor with one hit. She is the only infected not playable by human players in Versus mode.
AI Director

The artificial intelligence of Left 4 Dead features a dynamic system for game dramatics, pacing and difficulty called the "Director". Instead of fixed spawn points for enemies, the Director places enemies and items in varying positions and quantities based upon each player's current situation, status, skill and location, creating a new experience for each playthrough. The Director also creates mood and tension with emotional cues, such as visual effects, dynamic music, and character communication.
Valve has termed this dynamic set-up "procedural narrative". In addition to the AI Director, there is a second Director that controls music. It was created as a way to keep the soundtrack interesting throughout the game. The music Director monitors what a player has experienced to create an appropriate mix. The process is client-side and done by a multi-track system. Each player hears their own mix, which is being generated as they play through the game; dead spectators will hear their teammates' mix.
A far simpler version of the A.I. Director was previously used for some key battles in Half-Life 2: Episode Two. Valve is looking for ways to apply the Director in their future games to make pacing and difficulty more dynamic.