The premise

Left 4 Dead 2 is set in the aftermath of a worldwide pandemic of a disease nicknamed the "Green Flu", which rapidly transforms humans

into zombie-like creatures and mutated forms that demonstrate extreme aggression towards non-infected beings. A few humans are immune to the disease, while some of those who are infected have no symptoms. The Civil Emergency and Defense Agency (CEDA) and the U.S. military create safe zones to attempt to evacuate as many survivors as possible..

Like the first game, the five campaigns in Left 4 Dead 2 take place across a story arc. Set in the Deep South, the story begins in Savannah, Georgia, and ends in New Orleans, Louisiana. The four survivors have to fight their way through hordes of infected, using safehouses along the way to rest and recuperate in order to reach extraction points.

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The new survivors

Left 4 Dead 2 features a new cast of four human survivors:

Coach (voiced by Chad Coleman), a stocky high school football coach with a bad knee (although

this does not affect gameplay)

Nick (voiced by Canadian singer Hugh Dillon), a pessimistic gambler and con artist;

Rochelle (voiced by Rochelle Aytes), a low-level production assistant for a local television station that was reporting on the pandemic;

and Ellis (voiced by Eric Ladin), a friendly and talkative redneck mechanic who often talks about his friend Keith and their many misadventures.

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The new survivors
Name	Gender	Old job
Nick	Male	Con-man
Rochelle	Female	low-level producer
Coach	Male	Highschool coach
Ellis	Male	Mechanic

The infected

While also referred to as zombies, the Infected are humans who have contracted a mutated strain of an infection, though neither the source nor nature of this "Green Flu" are made clear in the games. The most numerous Infected encountered by the survivors are the common Infected. Though individually weak, they can swarm and overwhelm the Survivors, especially when separated from their teammates.

A new addition to Left 4 Dead 2 is the Uncommon Infected unique to each campaign. By virtue of location and equipment worn pre-infection, they possess abilities that separate them from the Common Infected.

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Uncommon infected
Type	Abilities
Riot	Immune to frontal damage
CEDA worker	Immune to flames
Fallen survivor	Drops loot
Clown infected	Attracts a small horde with it

The special infected

As in the first game, there are special Infected whose mutations grant them special abilities that make them highly dangerous, which act as

bosses. As in the first game, an artificial intelligence system called the AI Director drives gameplay by procedurally spawning enemies, weapons and items based on players' performance during any given campaign. In Left 4 Dead 2, the Director has been improved to encourage more participation by players, forcing players through difficult gauntlets to reach safety. It also has the ability to alter elements of the level, such as placement of walls, level layout, lighting, and weather conditions, making each play session unique. The Director now rewards players for taking longer or more difficult paths through each episode by providing useful equipment, such as special ammo packs or higher tier weapons, along these riskier paths.

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The new special infected
Type	Ability
Charger	Charges and pins down a survivor
Jockey	Jumps on top of and steers a survivor to danger
Spitter	Spits green fluid onto the ground, doing damage to survivors standing on it