Game WQ : L4D

Pennsylvania suffers an outbreak of "Green Flu"; a highly contagious virus causing extreme aggression, mutation to the body cells, and loss of higher brain functions. Two weeks after the infection four immune survivors— Bill, a Vietnam veteran; Zoey, a college student; Louis, an IT analyst, and Francis, an outlaw biker— make their way through the city of Fairfield, only to discover that the virus is creating more dangerous mutations. After narrowly avoiding these new infected, along with hordes of others, the survivors are alerted to the presence of an evacuation point at the nearby Mercy Hospital roof by a passing helicopter. Fighting their way through the city's streets, subway and sewers, they are rescued from the hospital's roof by the pilot, only to discover he is infected.
With Zoey forced to kill the pilot, the helicopter crash lands in an industrial district outside the city. Finding an armored delivery truck, the group uses it to make their way to the town of Riverside. However, they find the road blocked, and travel the rest of the way on foot. After an encounter with an infected lunatic in the local church, they discover that the town is overrun, and decide to head to a nearby boathouse for rescue. Contacting a small fishing vessel, they manage to reach the city of Newburg on the other side of the river, only to find much of it in flames. Seeking cover in a large greenhouse, their rest is interrupted when a military C-130 Hercules passes overhead, leading the survivors to travel through the city's business district towards Metro International Airport. Upon arrival, the group see that in an attempt to contain the infection, the military had bombed the airport while infected pilots crash planes in an attempt to land; the runway however is largely intact, allowing the survivors to fuel up and escape in a waiting C-130.
Despite this apparent rescue, it crashes as well, and the survivors once again find themselves alone on the outskirts of Allegheny National Forest. Following a series of train tracks through the area, the group find themselves at a functioning, but abandoned, military outpost. After answering a radio transmission, the survivors make their final stand against hordes of infected, before a military APC arrives to transport them to Northeast Safe Zone Echo, supposedly the only uninfected area yet to be overrun. Instead, they are kept at a military installation and informed that even though they are immune, they still carry the virus. They are temporarily held by the military before the base is overrun with infected. The four escape via train and travel south at Bill's insistence; Bill believes that they can find long-term safety from the infected on the islands of the Florida Keys.
At the portside town of Rayford, they find a boat but must raise an old rusty bridge powered by an aging generator to get the boat into open waters, assured that the machinery noise will alert a large horde. When Louis is injured and unable to move quickly, Bill sacrifices himself against the horde to make sure the generator keeps running and allowing the three to stay clear of the horde. After waiting for the horde to disperse, the three remaining survivors are taking time to mourn Bill's death when they encounter the four survivors from Left 4 Dead 2, who need to cross the bridge to continue to the west. Zoey, Louis, and Francis help the four to get across and fend off another horde as they lower the bridge, and then set sail for the Keys.

Left 4 Dead is a first-person shooter. In campaign mode, the player takes control of one of the survivors; if four human players are not available, then the remaining survivors are AI-controlled bots. They play through the levels fighting off the infected—living humans who have been infected with a rabies-like virus that causes psychosis The survivors are carriers of the disease, so they do not show signs of any symptoms.
The game is focused on cooperation and team play and thus eschews some "realism" conventions usual in most FPS games of the wider genre; colored outlines of teammates are visible through walls to help players stick together and coordinate their movement. If a survivor falls off a ledge, then they may automatically hang onto it and can only be helped up by another survivor. If a survivor's health is depleted, then they become incapacitated and can only be revived by another survivor, at which point they continue playing with a low amount of health that decreases over time. If a survivor has been incapacitated and revived twice without tending to their wounds, then they will experience distorted black-and-white vision, and the next incapacitation will kill the character. If a survivor takes enough damage while incapacitated, or is not eventually helped up by teammates, then the incapacitated character will die. During "Campaign" mode, if a survivor is killed, then they will respawn in a closet or other enclosed space after a period of time (except during key points in the scenario), but must be freed by another survivor to rejoin the team. Otherwise, the player must wait until the next level. However, if all human player survivors are killed or incapacitated, players will have to restart from the last checkpoint. Survivors can share first-aid kits and pain pills and help each other heal. Left 4 Dead has friendly fire (which causes no damage on the easy difficulty mode), increasing the need for caution around other survivors.
The survivors communicate by voice commands that are accessed by quick menus, and some may sound off automatically when performing actions such as reloading or spotting infected. Over 1,000 unique lines have been recorded for each survivor. Additional communication of player actions is conveyed through character lights. Also, weapon-mounted flashlights and muzzle flashes help the players in determining whether their companions are shooting, performing melee attacks, reloading or moving. 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.
The game is experienced through five campaigns that take place in various urban and rural locales. Multiple visual in-game hints—including license plates, park signs, markings on airport equipment, and lines of dialogue spoken by the survivors—suggest that these locations are in Pennsylvania, and similarly a memorial wall giving names of those who have died in the infection (actually names of the game's development team) along with their dates of birth and death suggests that the game takes place in October 2009. Each campaign is divided into five chapters (except Crash Course, which has two chapters) marked by safe rooms, which are checkpoints where players can heal, re-arm and revive characters who were killed.Specifically, the four campaigns are: "No Mercy", an urban setting culminating in a hospital skyscraper; "Death Toll", a small-town and countryside setting; "Dead Air", an airport setting; and "Blood Harvest", a woodland and farm setting. A two-level campaign, "Crash Course", was released on September 29, and is set in the outskirts of a small industrial town. The levels have distinct beginnings and ends, but there are a number of alternate routes to follow with more supplies, helping to create a sense of non-linearity. In the final chapter of each campaign, the players must defend a position from an onslaught of infected until rescue arrives. Each campaign typically lasts between 20 and 75 minutes depending on the difficulty level. Both platform versions of the game utilize an achievement system.