Left 4 Dead

Left 4 Dead begins in Pennsylvania, following an outbreak of so-called "Green Flu", a highly contagious virus causing extreme aggression and loss of higher brain functions.
Two weeks after the infection of Patient Zero, four immune survivors of the outbreak (Francis, Zoey, Bill, and Louis) make their way through the city of Fairfield, only to discover that the virus is creating new, even 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 by a passing news helicopter. Fighting their way through the city's streets, subway and sewers, the survivors are rescued from the hospital's roof by the pilot, only to discover that he is succumbing to the infection. ("No Mercy") With Zoey forced to kill the pilot, the helicopter crash lands in an industrial district outside the city. Finding a delivery truck that has been armored by other survivors, the group decides to use it to make their way to the town of Riverside ("Crash Course"). Before arriving in the town however, they find the road blocked, and are forced to travel the rest of the way on foot. Following an encounter with an infected madman in the local church, they discover that the town is overrun, and decide to head to a nearby boathouse for possible 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 ("Death Toll").