The Survivors

The Survivors are people in the world who have not yet been infected, however the term is commonly used to refer to the four playable uninfected humans.

Playable Survivors
Each playable survivor has his or her own unique appearance, personality and backstory, making them easier to identify at a distance. Players can choose which Survivor they control in the lobby, or be automatically assigned to a Survivor that is not being controlled by a player when they join a game that is already in progress.

Non-playable Survivors
As well as the four playable survivors there are also known to be many more. All remain unseen and most remain unnamed; however they are evident by graffiti left around most of the chapters, mainly in the safe houses. In some graffiti it is also stated that there were more survivors who were immune. The only other survivors you encounter are only spoken with, either through radio in an evacuation finale, or in the case of the "Church Guy" who locks you out of the safe room and tries to kill you by attracting a horde.

Evacuation Personnel
During the finale of each scenario, a survivor in a vehicle must arrive to rescue the four main characters from their evacuation point. These survivors are never seen, as they are usually concealed behind the windshields of their vehicles, excluding the helicopter pilot from the No Mercy campaign. These survivors include the aforementioned helicopter pilot from No Mercy, the boat drivers (John and Amanda Slater) from Death Toll, the C-130 pilot from Dead Air, and the APC driver from Blood Harvest. In all cases you communicate with them via radio.

There Are audio files within the Left 4 Dead File Folders That explain why the helicopter Pilot from No Mercy was running late, He atteampted to pick some more surviors on the street and was bittten by an infected and at the end of the audioclips its implied he turns into an infected himself.

"Church guy"
A man whose voice clips were found in the demo's files. He can be seen in chapter 3 of the Death Toll campaign, The Church. He appears to be afraid of infection, locking himself in the safe room that the Survivors must get through. It is later revealed that he was bitten by his friend, who became an Infected, and deemed himself immune to the virus after waiting an hour. Ultimately, he also becomes an Infected. His paranoia of the Infected drove him to insanity. He often says "better safe than sorry" and causes a mini-finale where he rings the church bell to attract a horde to kill the Survivors. After the mini-finale, the safe-house door can be opened and a special infected will be in there providing a conclusion to his change (he will never turn into a tank or witch, though there has been some rumors about turning into a Tank). His voice actor, Nathan Vetterlein, also voiced the Scout from Valve's Team Fortress 2.

Trivia

 * The Survivors were changed from their original looks to the new look to make them appear like someone you may encounter in real life, to make them look more "realistic."
 * It seems possible that each scenario is geared towards being the "home" of one of the four Survivors.
 * Bill makes a comment about the trailer in Blood Harvest, asking Francis if it reminded him of home.
 * Louis seems to know somewhat trivial pieces of information involving the sewer system of Riverside.
 * Zoey said that she always liked the main street in Riverside, and also, in some early Left 4 Dead articles, she is referenced as being from a farm, indicating a possible relation to Blood Harvest.