Left 4 Dead - Church Guy

The Church guy is a mentally unstable man who can be found in chapter three of the Death Toll campaign, The Church. He appears to be afraid of Infection, barricading himself in the safe room that the Survivors must get through. It is later revealed that he was bitten by a female survivor who claimed to be immune, yet became an Infected; Church Guy deemed himself immune to the virus after waiting an hour. As a result he suffers from extreme paranoia of Survivors, and it has obviously driven him to insanity. Ultimately, it turns out the Church Guy isn't immune after all; he will change into a Hunter, Smoker, or Boomer. He repeatedly says "better safe than sorry" and causes a Crescendo Event where he rings the church bell to attract a horde to kill the Survivors, apparently viewing this as a way for them to "prove their immunity/humanity". After the event, the safe-house door can be opened and a Special Infected will emerge, providing a conclusion to his change. You may hear him start to growl (Hunter), cough (Smoker), or gurgle (Boomer) 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 male, judging by the audio files. However, you can see a corpse of a female slumped against the wall in the safe room when you walk in. It does not appear to be a Common Infected, 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. Despite ongoing rumors and popular belief of a slim chance of occurring, he will never turn into a Witch or a Tank after the Crescendo Event. 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% each of either 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 or Witch unless there is a serverside mod in use. Alteration of the population.txt file to force the Church Guy to transform into a Tank or Witch can create very strange results, thus verifying the fact that the Church Guy will never spawn as a Tank or Witch.

Source: Left 4 Dead Wikia