Rescue Closets

Unlike in many games, when Survivors' health in Left 4 Dead drops to zero, they are incapacitated. If they aren't rescued, they bleed out. Survivors may also be thrown from high places (generally by something like a Tank). Regardless of the method, it all leads to the same thing: death. If the person that died was a human player, they enter spectator mode, which allows them to watch their teammates actions. This was implemented into the game so that the people who died would be able to watch how their living teammates survived and learn how to react next time. Yet, in the event of a Survivor dying, there's still a chance. Once the others have traveled a certain distance from their body, the Survivor will respawn in a room (this does not happen in Versus mode), usually a closet or other small room.

However, the Survivor can't get out on his or her own, wherever they spawn. They must be found and rescued by another person. The Survivor will be outlined in an orange aura, and they will yell for help. When the door has been opened, they will rejoin the team with a random tier 1 weapon and 50% health.

When a player rescues a Survivor from a closet for the first time on either version of Left 4 Dead, they get the achievement Hero Closet.

Tips

 * If you are the last Survivor, move to a safe location and attempt to wait things out. Once you see the orange outline and hear the respawned Survivors shouting, run to the closet they're in (all Survivors will respawn in the same closet) and rescue them. However, if you're closer to a safe house, go there and close the door behind you. When the next chapter starts, everybody will be back.
 * If the last Survivor dies, everybody will restart at the chapter's starting safe room.
 * If you die in single player mode, you are given no chance to respawn, and the chapter is restarted from the beginning safe house. You can only respawn if there are other people playing.
 * Survivors do not respawn in rescue closets in Versus mode.