Left 4 Dead is a cooperative first-person shooter video game. It was developed by Turtle Rock Studios, which was purchased by Valve Corporation during development. The game uses Valve's proprietary Source engine, and is available for Windows, Xbox 360 and Mac OS X. Development on the game was completed on November 13, 2008, and was released digitally on November 17, 2008, and at retail on November 18, 2008, in North America, In Australia on November 18, 2008, and on November 21, 2008, in Europe and Japan.
Pennsylvania suffers an outbreak of "Green Flu"; a highly contagious infection causing extreme aggression, mutation to the body cells, and loss of higher brain functions. Two weeks after the first infection four immune survivors—William "Bill" Overbeck, 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 him, 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 not yet overrun. Instead, they are kept at a military installation and informed that even though they are immune, they still carry the infection. 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. However, the generator gives out. Bill sacrifices himself in order to restart it, so that the others may reach safety. After waiting for the horde to disperse, the three then encounter four more survivors. They move the boat to the other side of the bridge and help them re-lower the bridge so they can cross in their car. Afterwards, Louis, Zoey, and Francis head back to the boat and set course to the Keys.