Health

"Hold on, I'm gonna patch you up!"

- Zoey "I need to find some Health!"

- Rochelle

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. Health points remaining at the end of one Chapter are carried over to the start of the next chapter. In Versus, the Survivors will always begin a chapter at full health.

Health points are progressively lost as a player receives damage or as time goes by (if Pain pills and Adrenaline is used).

When a Survivor's total health is 39 or less, the character begins limping and their walking movement speed decreases from 220 Hammer Units (4.19 m/s or 13.75 ft/s) to 150 Hammer Units (2.86 m/s or 9.38 ft/s) per second. An incapacitated player who has been revived but not healed by a First aid kit will decrease to zero in the space of 37 seconds after reaching 1 point of health and suffer a further walking movement speed degradation to 85 Hammer Units (1.62 m/s or 5.31 ft/s) per second.

Crouching movement speed is unaffected by any of the above health damage constraints, always at a fixed 75 Hammer Units (1.43 m/s or 4.69 ft/s) per second.

In Versus mode, the movement speed of the Special Infected do not change, regardless of their current health. Also, any special infected being controlled by the AI in Versus mode will not display their health or other information on the HUD.

Incapacitation
"Shit, I'm down!!"

- The Survivors

Once a Survivor's health has depleted to 0, that character is incapacitated and falls to the ground, at which point 300 temporary health is given (rarely above 300) and that person must be helped up by a teammate; failing that, bleeding out and death will ensue.

Incapacitated Survivors only have access to a Pistol (two if they were dual-wielding before incapacitation) for defense. If an incapacitated player was using a melee weapon at the time of going down, it will be changed into a P220 Pistol. An incapacitated player's pistol will have degraded accuracy, rate of fire, and reloading speed (excluding Magnum Pistol where it retains its accuracy and rate of fire). No melee shove can be employed whilst incapacitated.

Friendly fire risks whilst incapacitated differs between Left 4 Dead and Left 4 Dead 2. It is left active in the former and teammates will suffer health loss from an incapacitated player's careless shooting. However, it is disabled in the later game and so incapacitated players can shoot freely without regard to their teammates' health.

Incapacitated players take reduced damage from Tank and Witch attacks and Spitter goo; but a pounced Hunter and Common Infected will inflict increased damage. An incapacitated team member suffering a Hunter attack is thus a priority rescue task.

Smokers, Chargers, and Jockeys cannot use their special attacks on an incapacitated Survivor; Smokers, Chargers, and Hunters won't release an incapped survivor until killed, shoved (except Chargers who cannot be shoved) or the survivor is dead. Meanwhile, the Hunter can also pounce on an incapacitated survivor while a Spitter can spew acid on incapped survivors, making them a huge threat.

Boomers are a serious problem as the incapacitated player may be covered in Boomer bile and surrounded by Common Infected, thus making it exceedingly difficult for teammates to effect timely rescue and resuscitation.

Incapacitation can occur when a player falls and hangs from a ledge. The selfsame reduced damage effects referred to above come into play, but they cannot defend themselves in any way. Health damage from hanging from a ledge is only temporary and the player recovers to normal status on being pulled back up by a teammate (refer to the the following section for advice on how this effect can be used to a player's advantage during gameplay).

Valve had planned to add an ability that allowed incapacitated players to crawl slowly (in fact, there is an unused animation regarding this). However, this was cut. Nevertheless, it's used in some modded servers.

Black/White Mode (Monochrome Vision) A.K.A. Third Strike
"Don't worry, I can help you. But we need to find you some First Aid, man. If you go down like this again, that's it for you."

- Zoey "Man, you look like one of those dudes in the driving school film... this may be it for you."

- Coach

When a Survivor has been incapacitated twice without being healed, the affected player's vision will turn black and white/grayscale with a notable audio heartbeat; item glows will be red and more intense; and depending on the player's graphical settings, the edges of their screen will become blurred, as if they were developing tunnel vision (Ellis may say that he's hurt so bad he can't see right). This is likely caused by hypovolemia, the rapid loss of blood.

Other alternatives might be extreme anger (due to large amounts of oxygen and adrenaline in the body) and extreme fear and distress. This signals a dire health situation requiring urgent healing. Unless this player is healed with a First-aid kit, they will die once their health depletes a third time. At this point, the only way to revive the dead survivor is to use a Defibrillator, reach the Safe Room, or free them from a Rescue Closet (except in Realism, in which Rescue Closets are disabled).

After being revived the second time, the Survivor will be left with only temporary health. Once that temporary health runs out, they will be left with only 1 unit. On any level of difficulty, one hit from any Infected will kill that Survivor immediately.

Bots will not notice if a Survivor is black and white. If their health is below the healing point, they will heal themselves first. The only thing to do is to either kill the Bot or, if they haven't grabbed the First-aid kit yet, shoot them a few times, run to the kit, and heal yourself ASAP.

If three Survivors are incapacitated and one is still alive, protect the player still standing at all costs if you don't want to restart the chapter!!! If the only standing Survivor is an AI Bot, it will go for the closest Survivor, not the one with the least health or in great danger in some situations. Even if you get angry that the Bot isn't helping you, don't try to slow it down. Letting it help another Survivor will speed up the process of saving you.

One method to utilize the ledge hang while in black-and-white mode is when you are extremely low on health (such as having only 5 units of health remaining) and a horde is on its way. A Survivor can hang from the ledge so the horde cannot drain the last of their health as the incapacitation temporary health of 350 will take the damage. Ledge-hanging can be used even in black-and-white mode without killing the player, as long as they are not the only Survivor still alive. Due to this, after the horde has subsided, chances are, the rescued player will be left with 1 unit of health, but better this than dying during it.

Note that Survivor Bots will pull the hanging Survivor back up, thus exposing the remnants of their temporary health to the horde, so it is best to do this when there are human players only and/or the Bots are all dead. With human players, notify them not to try and help you during the horde to make this easier, though remember you are not invulnerable and so if your teammates fail to do a satisfactory job of protecting you while you hang, you will find yourself plummeting rather than climbing back up.

Also note that your team will have one gun less to fight the Infected, which on Expert, can be fatal to them. Only use the ledge hang method if the team is well coordinated on Expert and even Advanced. Doing this on Normal or Easy with a less coordinated team will be less of a problem due to the Infected's low damage attacks.

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

- Bill "Stay close, we're a party of two now."

- Coach

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. In one of the Mutations, Death's Door, death will occur as soon as a Survivor's health reaches zero since incapacitations are disabled. However, you can still hang on a ledge and have temporary health until you are pulled back up.

If all Survivors perish during the course of the game in Campaign, Realism, or if the player dies in Single Player, then the level they were on will reload with everyone back at the last safe room, with their health, weapons, and items restored to what they had when they first entered that particular safe room during the current game. In Versus, Realism Versus, and Scavenge, control of the Survivors and Infected is switched between the two teams. In Survival, the Survivors simply start a new game with everyone alive and at full health with First aid kits and Pain pills.

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 (or more) Survivors are incapacitated and the last is dominated by a Smoker, Hunter, Jockey, or Charger, the incapacitated Survivors can still save the other Survivor with their Pistols/Magnums and it won't count as a loss, if saved before the last Survivor is incapacitated.
 * When a Jockey incapacitates a Survivor, he will leap off, allowing the Survivor to use their sidearm to help teammates (if within range). This often happens when the Jockey rides on a Survivor with low health, incapacitating the Survivor faster and not be ridden away.
 * The game is only over when every Survivor is either dead or incapacitated. There are still things that can kill the Special Infected or force them to release the Survivor in this situation, such as an explosion (a exploding Boomer, an item that is already been shot or on fire, a thrown Pipe bomb, or a grenade that is already in the air), a "friendly" attack from a Tank, or fire that is already burning on the Special Infected.
 * Hanging from a ledge counts as incapacitation. So if three Survivors are hanging from a ledge and the fourth Survivor's health reaches zero, incapacitating them, the game will end.
 * Dying in Single Player mode, leaving the Survivor Bots to play alone.
 * Dying in Campaign, Realism, Versus, Realism Versus, Scavenge, and Survival with all the other Survivor human-controlled players, leaving only Survivor Bots alive.
 * However, this will not be the case if  is set to 1. If there are no human players alive, the Survivor Bots will run through the map on their own. If they find a human player in a closet, or reach a safe room, the game will continue as normal with the human player leading the Survivor Bots again. Note that Bots struggle to trigger Crescendo Events and effectively run through a Rolling Crescendo on their own.

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

- Rochelle "Cover my ass! I'm healing!"

- Francis

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 Shot 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 time.
 * All forms of temporary health reduce at one point per 5 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 first-aid kit. It is a good idea for a Survivor with the least health to carry it:
 * 1) Because they have no health pack to drop
 * 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.
 * In the Mutation, Bleed Out, players only have temporary health. However, Pain pills and Adrenaline Shot can bring them back from Black/White Mode, but however, the heartbeat sound stays. (In most modes, you need a First-aid kit.)
 * The Mutation, Healing Gnome, has a gnome players must escort from level to level and it recovers a Survivor's health while being held by restoring 1 health point per half second. This is the only way in the entire game that a Survivor can recover health that does not involve the use of healing items and is also the only way a Survivor can regenerate health over time, whereas it cannot be done anywhere else.

Achievements

 * See Main Article: Achievements

Left 4 Dead

Left 4 Dead 2