Health

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

- Zoey

"I need to find some Health!"

- Rochelle

HP (also known as health) is the hit point total that governs the survivability of the Survivors and Infected in Left 4 Dead and Left 4 Dead 2. 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 lost as Survivors take damage from falling from a high enough distance, attacked by infected, friendly fire (with the exception of Easy difficulty), or standing in fire. In the case of health obtained by Pain Pills or Adrenaline, it will slowly trickle down. This is the same case for Survivors revived from incapacitation.

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.

A survivor's speed will always rise up to 260 Hammer Units per second whilst under the effects of Adrenaline, regardless of their health before taking the shot.

However, the mechanics are slightly different when the Survivors are introduced to water, this can commonly be seen in the Hard Rain campaign. When a Survivor is in water that is up to their knees their speed changes from 220 Hammer Units to 115 Hammer Units per second, this is similar when a character's health is 39 or less, changing their speed of 150 Hammer units to 115 Hammer Units per second. At 1 health point with a movement speed of 85 Hammer Units per second will remain unchanged.

Furthermore, when the Survivors are in water that is deemed as swimmable (usually up to their waist), their speed differentiates based on certain factors. Movement speed changes from 220 Hammer Units to 92 Hammer Units per second, and is similar when the Survivor's health is below 39 health points, and their speed changes from 150 Hammer Units to 92 Hammer Units per second. When the Survivor is at 1 health point their movement speed changes from 85 Hammer Units to 68 Hammer Units per second. Movement speed whilst crouching is changed from 75 Hammer Units to 60 Hammer Units per second. Finally, whilst under the effects of Adrenaline their speed changes to 208 Hammer Units per second. The Survivor's forward velocity does, of course, change if they move their camera to an angle looking up or down since they are in water that they can swim in.

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

Incapacitation
"Help I'm down!!"

- The Survivors

Once a Survivor's health has depleted to 0, that character becomes incapacitated and falls to the ground with 300 temporary health (if some temporary health remained before you got hit, they may get added to the 300 health total). The incapacitated Survivor will need another non-incapacitated Survivor to help them up, or else they will eventually bleed out and die. The health will slowly tick down by 3 until death unless someone is in the process of reviving them.

Incapacitated Survivors only have access to their secondary weapon for defense. In the case that the Survivor used a melee weapon at the time of going down, it will be temporarily changed into a P220 Pistol. An incapacitated player's pistol will have degraded accuracy, rate of fire, and reloading speed (excluding the Magnum Pistol where it retains its accuracy and rate of fire). A melee shove can not be performed by an incapacitated survivor.

Friendly fire mechanics 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 so incapacitated players can shoot freely without regard to their teammates' health.

Incapacitated players take reduced damage from Witch attacks and Spitter goo; whilst a Tank (aside from Versus), a pouncing Hunter and the Common Infected will inflict increased damage. An incapacitated team member being pounced by a Hunter is thus a priority rescue task. However, Tanks can still pose a major threat to incapacitated players if they have access to movable props such as cars or dumpsters to block any attempt to rescue said players. If a Tank knocks such an object into an incapacitated Survivor, the attack will inflict massive damage and possibly block other survivors from helping.

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

Boomers are a serious problem as the downed player may be covered in Boomer bile and surrounded by Common Infected or explode next to the Survivor trying to revive them, thus making it exceedingly difficult for teammates to make a timely rescue and resuscitation.

Incapacitation can occur when a player falls and hangs from a ledge. The reduced damage effects referred to above come into play, but hanging players 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 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). Despite being cut, however, it's used in some modded servers.

When about to fall from a high location, the Survivor will grasp the ledge, which counts as incapacitation (you will not, however, be able to use your sidearm while holding the ledge). Note that all 4 players cannot be incapacitated by hanging from a ledge, as the last Survivor will not grasp the ledge and will instead fall to their demise.

When an incapacitated Survivor is revived, they'll be left with 30 temporary health points. This is usually a good time to take Pills or an Adrenaline Shot to keep your health up. However, keep in mind that all of your health is only temporary, and will eventually lower again to the point of needing more health. If a Survivor goes down again and is revived for the second time, their vision will turn black and white (more on this below).

Final Incapacitation
"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 might mention "[He's] hurt so bad [he] can't even see right"). This is likely caused by hypovolemia, the rapid loss of blood. Other alternatives might be extreme agitation, either due to pain, or anger and/or extreme fear and distress due to the fight or flight response, which warps the character's perception even if not their actual visual acuity.

This signals a dire health situation requiring urgent healing. Unless the player is healed with a First Aid kit, they will die once their health depletes a third time. If this were to happen, at that point, the only way to return the dead Survivor to the game is to use a Defibrillator, reach the Safe Room, or free them from a Rescue Closet (Realism is an exception to this, in which Rescue Closets are disabled).

When revived from incapacitation, the Survivor's 30 gained health is only temporary (mentioned previously above), and when the temporary health lowers down to 1 again, any damage from infected, friendly fire, standing in fire or falling will instantly kill that Survivor.

When playing with bots, they will not know when a Survivor is black and white. They only attempt to heal Survivors with low health, meaning if another teammate is low on health but not black and white, the bot might go to heal them instead, even though it's advised to heal the black and white player first. To stop this problem from occurring, try to grab first aid kits before the bots can and/or hold onto your first aid kids and let the bots use theirs first, giving you or other players the ability to heal the Survivors who need it most first.

If there is only one Survivor standing with everybody else incapacitated or killed, it is of utmost priority to cover that Survivor, due to that Survivor being the only one capable of reviving the rest of the team. If the only standing Survivor is an AI Bot, it will go for the closest Survivor and not the one with the least health or in great danger in some situations. Do not try to slow it down by any means, as letting it help another Survivor will speed up the process of saving another player, which will still increase the odds of avoiding a team wipe.

Hanging/Clinging
Hanging or clinging is when a survivor falls off a ledge to a big fall. This will cause the survivor to hang off the ledge and the bleed out timer will appear. Unlike incapacitation, you are fully helpless and can't use a pistol. To bypass this, hold the jump button.

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 lying on their back with their arms spread apart. Dying does not mean that the player has been eliminated from the game; they can simply wait one minute until transferred to the nearest available rescue closet, and then have another Survivor rescue them.

In Left 4 Dead 2, alongside closets, Survivors can also be revived by using a Defibrillator. When a 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 made so when revived from death, the survivor will have something to use, as the game is not programmed to work correctly when a survivor has no items.

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 are either dead or incapacitated during the course of the game in Campaign, Realism, or if the player dies in Single Player, then the chapter they were on will restart, 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 are switched between the two teams. In Survival, the Survivors simply start a new round with everyone alive and at full health with First Aid Kits and Pain Pills.

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 standing is grabbed by a Smoker, Hunter, Jockey, or Charger, the incapacitated Survivors can still save the other Survivor by killing off the special infected with their Pistols/Magnums and won't end or restart the chapter if the last Survivor was saved before incapacitation.
 * Once a Jockey incapacitates a Survivor, he will jump off, allowing the Survivor to use their sidearm to help teammates (if within range). This often happens when the Jockey leaps 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, meaning there are still things that can kill the Special Infected or force them to release the Survivor nearing incapacitation or death, such as an explosion (an exploding Boomer, an item that is already been shot or on fire, a thrown Pipebomb, 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, meaning if three Survivors are hanging from a ledge and the fourth Survivor's health reaches zero, the game will end.
 * All Player-Survivors dead in Campaign, Realism, Versus, Realism Versus, Scavenge, leaving the bots alone.
 * 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 start Crescendo Events and effectively run through a Rolling Crescendo on their own. They can even teleport through a Crescendo item to proceed with the chapter.
 * This can also be changed for the campaign versions to continue with only bots alive in the second game with an add-on in the Steam/PC version (it works online).

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

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

- Francis


 * Main article: Healing items

Health can be restored in various ways:
 * Using the 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 (3 seconds in Left 4 Dead 2).
 * 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 because:
 * They won't (or shouldn't) have a health pack to drop.
 * 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 that 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
Left 4 Dead

Left 4 Dead 2