Health

"No shame kid, I would have pissed my pants too, but if we don't find you some first aid, you aren't gonna survive another hit like that."

- Bill Health is the hit point total that governs the survivability of the Survivors in Left 4 Dead. All Survivors begin with a total of 100 health at the start of every campaign, and when they complete a chapter their health will be saved for the next one. In Versus the Survivors will always begin a chapter at full health.

As the Survivors receive damage, their health decreases. When a Survivor's total health is 39 or less, they begin limping and their walking movement speed decreases from 220 to 150. If a Survivor has been revived from incapacitation but has not been healed by a first-aid kit, then at the moment their health would decrease to zero (3.7 seconds after reaching 1 point of health), that Survivor's walking movement speed further decreases to 85. Crouching movement speed is not affected by any of these speed decreases.

Incapacitation
"Shit, I'm down!!"

- The Survivors

Once a Survivor's health has depleted to 0, they will count as incapacitated and will fall to the ground, but still be able to use their pistols. When incapacitated, a Survivor has 300 health and must be helped up by teammates, or they will bleed out and die. Survivors also take reduced damage from some attacks when incapacitated but the hunter will have increased damage.

When a Survivor has been incapacitated two times without being healed, their vision will turn black and white with a notable audio heartbeat to represent their current situation and item glows will increase and be red. If they fall for the third time without being healed with a First aid kit, they will be killed rather than incapacitated the next time their health reaches 0. Incapacitation can also happen when a Survivor hangs from a ledge - the same effects of reduced damage happen, but they do not count towards a black and white situation nor can you use your pistols to defend yourself.

Death
"This is going to hell in a hand basket real fast!!!"

- Bill

Upon death, the deceased Survivor will drop all of their weapons and items and turn into a ragdoll. In Left 4 Dead 2, however the Survivor dies with their arms spread apart and lying on the back. Dying does not mean that the player has been eliminated from the game, they simply need to wait one minute until transferred to the nearest rescue closet, and then have another Survivor rescue them. In Left 4 Dead 2 Survivors can also be revived by using a Defibrillator rather than a closet. When death occurs in Left 4 Dead 2, the Survivor will drop all items except their secondary weapon, whether it be Pistols or a melee weapon. This was most likely done so that if a player is revived through a Defibrillator, the player will have something to defend themselves with as they get up and will also have something in case their main weapon was taken. In Versus and Realism mode, the Defibrillator is the only option for Survivors to return during a level, as they cannot come back to life inside rescue closets. Otherwise they will have to wait until the Survivors are defeated, finished the chapter, or have restarted the round.

However, there are several other ways to lose the game related to death:
 * Being fully overcome by the Infected, leaving the group unable to play. However, if one Survivor is incapacitated and another is dominated by a Smoker, Hunter, Jockey, or Charger, the incapacitated Survivors can still save the other Survivor with their Pistols and it won't count as a loss.
 * Dying in Single-player mode, leaving the AI Bots to play alone.
 * Dying in Campaign or Versus mode with all the other Survivor human-controlled players, leaving only AI Bots alive.
 * If all human-controled survivors die leaving only the AI alive, the round or chapter will end.

Healing
"Stop moving... I don't want to mess this up."

- Rochelle

Health can be restored in various ways:
 * Using a first-aid kit removes all temporary health, and then restores 80% of the player's current missing health as permanent health.
 * In other words, if you have $$x$$ permanent health, using a first-aid kit grants you $$0.8(100-x)$$ additional health.
 * No matter how much your health is, when you heal your health will ALWAYS be either at 80 or above.
 * This can technically reduce your health total if you have enough temporary health. For example, if you have 50 permanent health and 50 temporary health, your total would be 100. If you then use a first-aid kit, then all of your temporary health is removed, and 40 of the 50 remaining points are healed, leaving you with a total of 90 permanent health, less than the original 100.
 * Using pain pills grants 50 temporary health, which will slowly decrease to normal health as time passes.
 * Using an adrenaline injector grants 25 temporary health. The adrenaline also makes you immune to melee fatigue and the horde's slowing melee attacks and allows the player to perform actions faster for a short period of time.
 * All forms of temporary health reduce at one point per 3.7 seconds.
 * When you take damage, your permanent health drains before your temporary health.
 * The Defibrillator can be used to bring a dead Survivor back to life. The defibrillator will take the space of the health pack. It is a good idea for a Survivor with the least health to carry it. 1) because they have no health pack to drop); and 2) because if they die another Survivor can pick it up and use it on them. The defibrillator looks like a lightning bolt when you are carrying it. When you are revived with the defibrillator, you will be given 50 permanent health points, similar to how you'd be given 50 permanent health points after being taken out of a rescue closet.
 * Being revived from incapacitation grants the Survivor 1 permanent health and 29 temporary health (giving a total of 30). Incapacitating a teammate in order to do this is not recommended as a form of healing, of course.
 * Being rescued from a rescue closet grants the Survivor 50 permanent health and Tier 1 Weapons. The type of weapon depends on what the Survivor was carrying when they were killed. For instance, if the Survivor had an Assault Rifle, that Survivor will get a Submachine Gun when they are rescued. This will not occur if the survivors are overwhelmed and restart the chapter.