Story

The events taking place in Left 4 Dead and Left 4 Dead 2 are part of a single story line, following two groups of survivors as they attempt to survive the zombie apocalypse.

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").

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 is shocked to see that in an attempt to contain the infection, the military had bombed the airport; the runway however is largely intact, allowing the survivors to fuel up and escape in the waiting C-130 ("Dead Air"). Despite this apparent rescue, a second crash occurs, 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 ("Blood Harvest"). 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. (The Sacrifice comic) 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.("The Sacrifice") 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. ("The Passing") 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 2
Like its predecessor, Left 4 Dead 2 is set two weeks after a worldwide pandemic of an infectious disease that rapidly transforms humans into zombie-like creatures and mutated forms that demonstrate extreme aggression towards non-infected (much like the ones in 28 Days Later). Few humans are immune to the disease, still carrying the infection but showing no symptoms. The Civil Emergency and Defense Agency (CEDA) and the U.S. military create safe zones to attempt to evacuate as many American survivors as possible. Left 4 Dead 2 introduces four new survivors—Coach, Ellis, Rochelle, and Nick—who are immune to the disease and have individual back stories that are provided through character dialog. Unlike the first game, which had no significant story development, the five campaigns in Left 4 Dead 2 are set across a story arc, set in the Southern United States, which starts in Savannah, Georgia, and ends in New Orleans, Louisiana. The four survivors have to fight their way through the hordes of infected, using safehouses along the way to rest and recuperate in order to reach extraction points.

After climbing to the roof of a hotel in Savannah to be rescued, the survivors find themselves abandoned by rescue helicopters, and so make their way to the local mall, after hearing word of a second CEDA evacuation point there ("Dead Center"). After a brief encounter with a gun store owner, Whitaker (voiced by Dayton Callie), the survivors discover that the mall is overrun and all of the CEDA agents have either been killed or have become infected themselves. The four then use a stock car to bust out of the mall and travel towards New Orleans. On their way there, they encounter the original survivors featured in Left 4 Dead at the portside town of Rayford ("The Passing"), before finding the highway completely blocked by wrecked vehicles. The survivors are then forced to travel through a still-operating (but abandoned) amusement park, and start a huge fireworks-and-lights show used by the rock band, The Midnight Riders, in order to attract the attention of a helicopter pilot ("Dark Carnival"). After being rescued, they realize their pilot has been infected. Nick is forced to kill him, which causes the helicopter to crash into a bayou ("Swamp Fever"). They find brief shelter in a plantation mansion and make radio contact with a boat captain named Virgil who can get them to rescue, but he needs additional diesel fuel to make it to New Orleans. Amid a torrential downpour, the survivors go ashore, make their way through an abandoned sugar mill to a gas station to get fuel supplies, and return to the boat ("Hard Rain"). Virgil takes them to New Orleans where the military appears to be evacuating civilians. The group head to the extraction point, but discover that the military is actually bombing bridges to cover their retreat ("The Parish"). The survivors then make their way across a lift bridge amidst a mass of infected and board the last military helicopter leaving the city before the bridge is destroyed by an air strike. Judging from their dialog, the military pilots suspect that the survivors are "carriers."

Although the survivors' fate after evacuation is left unexplained after the bridge finale, Chet Faliszek, the game's writer, said that the military is taking survivors to cruise ships in an attempt to escape the worldwide infection.