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 humans who are immune to the Infection although a comment by Bill suggests that they could become an Infected at any point. Fortunately, this does not happen during the game. It is never explained thus far how these people all have met each other.

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 except the chopper pilot 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 windscreens 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. It also indicates more Survivors in Blood Harvest when contacting the driver of the APC. After you answer the radio, one of the radio communicators yells back to another one. One of the two is presumably the driver.

"Church guy"
A crazy man who can be found in chapter three 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 claimed she was immune), who became an Infected, and deemed himself immune to the virus after waiting an hour. Ultimately, he also becomes an Infected. He is suffering from extreme paranoia of the Infected, and it has obviously driven him to insanity. He often says "better safe than sorry" and causes a crescendo event where he rings the church bell to attract a horde to kill the Survivors. After the event, 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 Witch or a Tank, despite popular belief). You can also hear him start to growl before you open the door. After opening the door and ascending to the second floor of the safe room, the Survivors can see that the Church guy has written his motto all over a wall, over 180 times. His voice actor, Nathan Vetterlein, also voiced the Scout from Valve's Team Fortress 2.

The Survivor who Infected him is a female, you can see a corpse of a female slumped against the wall in the safe room when you walk in. It doesn't appear to be a zombie you fight, but a body that spawns there to set the mood like the dead soldiers on the roof of Mercy Hospital. However, this may be a slip up by the creators, or she could have been killed by biting him.

There has been ongoing rumours that there is a very slim chance of the Church Guy transforming into a Tank after the mini-finale event. However if players were to examine the left4dead/scripts/population.txt file, it appears that there is a 50% chance of a Boomer appearing, and 25% of a Smoker or a Hunter appearing. No mention of the Tank or the Witch are included in this file, making it clear that there is no chance of the Church Guy becoming a Tank unless there is a serverside mod in use. Alteration of the population.txt file to force the Church Guy to transform into a Tank can create very strange results, thus verifying the fact that the Church Guy will never spawn as a Tank.

Chicago Ted
Chicago Ted is an evidently egotistical survivor, known for scrawling "No zombie is safe from Chicago Ted" on the wall of a safe room. Whether or not he's still alive is unknown. He presumably originates from Chicago, Illinois.

Distant Survivors
In the campaigns Dead Air and No Mercy, occasionally, a distant gunshot, explosion, or gun burst may be heard. It can only be assumed that these are other Survivors. As they are infrequent, however, it is likely that few of them survive. Not to say that none of them do, as, from The Sewer, looking very closely at the roof of Mercy Hospital in the distance, the helicopter can be seen flying in and out, hinting at more than one evacuation. The helicopter pilot will also sometimes comment to the Survivors, "It looks like you'll be my final run of the night.", further indicating that he had been saving other Survivors from the rooftop.

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". Survivors in Left 4 Dead 2 started from this, but still, have changed from the first screenshots and videos(everyone but Nick).
 * 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, although this is most likely an insult to Francis saying that he is poor.
 * 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. She also stated that she had been to Newburg before.
 * Chicago Ted is a meme on Internet forums, assumed as being the best of the best in a zombie outbreak.
 * In the beta of Left 4 Dead, Zoey and Francis show some sort of affection towards each other, this proved to be too distracting, so it was removed.
 * The beta of Zoey looks like Chell from the popular Valve game Portal.
 * Chicago Ted's graffiti is based on a piece of graffiti found by Erik Wolpaw and Sean "Seanbaby" Reilly
 * The 3 male survivors in L4D2 all bear resemblances to characters in the Dawn of the Dead remake; the Coach being Kenneth, Nick being Michael, and Ellis being Terry.