Difficulty

There are four difficulty levels in Left 4 Dead. Each level refers to the difficulty of the campaign, and each has drastic differences over the next. For example, on the Easy difficulty, it's rare to see a horde outside of the occasional Crescendo Event, whereas on Expert, it's likely you'll face a horde around every corner. The specific differences between each difficulty level are listed below.

Easy
As the name suggests, this difficulty is the easiest of the four. This mode would be recommended for beginners and those who simply want to relax. This is also an easier way to get achievements without the trouble of the harder difficulties.

If you feel extremely lazy, it can take about 50 minutes to 1 hour to complete if you keep healing allies when they don't need to, and using the throwable items that you can find.

Several Special Infected can be dangerous, such as the Charger, Jockey, and Hunter even though they barely do any damage, when surrounded by Commons, it can be lethal and cause you to restart from the saferoom.

Details

 * The Tank has 3000 health points, does 24 damage per hit and 75 when pounding.
 * The Witch has 1000 health points and is startled in 8 seconds. (if you want to delete, please do a test on Easy difficulty first!)
 * Friendly fire does not inflict damage, but friendly fire incidents will still register and disqualify players from earning the Safety First achievement in Left 4 Dead. The damage will also be count into Steam player's statistics on PC.
 * Molotovs, Gas cans, Fireworks, and Explosive Barrels, once ignited, can still cause damage to you and other Survivors, so watch where you throw or shoot them. Note that a Survivor Bot can accidentally hit Gas cans, Fireworks, Explosive Barrels and deal damage to you and the team. The worst thing that can happen is the fire incapacitates you, then bots run into the fire to save you, causing them to get incaped if low on health.
 * Pipe Bomb, Oxygen tank, Propane tank, and Grenade Launcher will not do damage to your teammates, but will do roughly 5 to 30 damage to yourself if detonated too close. A Survivor Bot can also accidentally shoot Oxygen tank or Propane tank and damage him/herself.
 * In The Sewer, Survivors can incapacitate or kill themselves by shooting the gas station.
 * Common Infected have 30 health and deal 1 damage per hit, 0.5 if attacking a Survivor's back.
 * There is roughly a 1/5-second grace period where the Survivor will not take damage after being inflicted upon. This only affects damage dealt by the Common Infected.


 * The Smoker deals 10 damage per hit on ensnared Survivors and has a 5 seconds reaction time before attempting to ensnare a Survivor.
 * The Hunter deals 10 damage per claw and 5 damage every half-second to pounced Survivors.
 * The Charger deals 5 damage in a punch, 10 damage when its target reaches a wall, and 15 damage per slam.
 * The Jockey does 3 damage per hit while riding a Survivor and does 1 damage per hit.
 * The Spitter's spit acid's average damage is 31 and the body acid's average damage is 32 and does 1 damage per hit.
 * Chances for Survival: 4 Survivors = 100%, 3 = 90%, 2 = 70%, 1 = 20-40%
 * Special Infected such as the Smoker will pause before attacking, and Special Infected usually don't sneak up on you.
 * Bots will know where the Special Infected are, espically Smokers.

Normal
Normal mode is fairly balanced, as far as the difficulties go. It's harder than easy, but not something that will quickly decimate an inexperienced player, such as in advanced or expert.

Normal is also the set difficulty for Versus mode and Survival mode. Differences between other modes and Versus will be stated in parentheses.

Details

 * The Tank has 4000 (6000 in Versus) health points, deals 24 damage per hit and 75 (24 for Versus) damage when pounding.
 * The Witch has 1000 health points and is startled in 5 seconds.
 * Friendly fire inflicts 10% of the weapon's damage to the Survivors.
 * Common Infected have 50 health and inflict 2 damage per hit in the front, 1 from the back.
 * There is roughly a 1/4-second grace period where the Survivor will not take damage after being inflicted upon. This only affects damage dealt by the Common Infected.
 * The Smoker does 10 damage per hit on ensnared Survivors, 2 damage per hit (and 3 while dragging in Versus).
 * The Hunter deals 10 damage per claw and 5 damage every half-second to pounced Survivors.It also does a certain amount of damage on a pounce in versus depending how far away and how high you pounce from.


 * The Charger does 10 damage in a punch, 10 damage when its target reaches a wall, and 15 damage per slam.
 * The Jockey does 4 damage per hit while riding a Survivor and 2 damage per claw.
 * The Spitter's spit acid's average damage is 68, and the body acid's average damage is 69 and does 2 damage per hit.
 * Average chances of survival: 4 Survivors = 80%, 3 = 60%, 2 = 40%, 1 = 20%

Advanced
Advanced is for more experienced players. Although it's not as devastating as Expert, it demands a high level of competency and care. If the player is not careful, then they should be ready for multiple tries per map, as things can get pretty hard — and friendly fire incidents are the bane of a serious player, as they take off quite a bit of health here. The fact that first aid kits don't spawn very often only serve to exacerbate the problem further, unless the AI director seems to know your whole team is low on health and will spawn some in the health holders. Also, Tanks can cause huge amounts of problems as Special Infected can possibly cause you to start all over again due to the Tank's high amount of health.

It takes 20 hits for Common Infected to incap you from the front, and takes 40 hits to incap from the back.

Details

 * The Tank has 8000 health points, does 50 damage per hit and 100 when pounding.
 * The Witch has 1000 health points and is startled in 5 seconds.
 * Non-headshots get no bonus damage.
 * Common Infected have 50 health and inflict 5 damage per hit from the front, 2.5 from the back.
 * There is roughly a 1/4 second grace period where the Survivor will not take damage after being inflicted upon. This only affects damage dealt by the Common Infected.
 * Friendly fire inflicts 50% of the weapon's damage to the Survivors. Be careful with firing!
 * The Smoker does 20 damage per hit on ensnared Survivors and 5 damage per claw.
 * The Hunter deals 20 damage per claw and 10 every half-second on pounced Survivors.
 * The Charger does 15 damage in a punch, 15 damage when its target reaches a wall, and 15 damage per slam.
 * The Jockey does 6 damage per hit while riding a Survivor and 5 damage per claw.
 * The Spitter's spit acid's average damage is 68, and the body acid's average damage is 69 and does 5 damage per hit.
 * Average chances of survival: 4 Survivors = 60%, 3 = 45%, 2 = 30%, 1 = 15%

Expert
Expert mode is the toughest difficulty of the four. It is not at all recommended for the inexperienced or impatient, as it may take many tries for the player to complete a single chapter. In this mode, never fear death, and always be on the move. Standing still is an almost guaranteed way to be killed here. But then even moving has its downsides. The player must be ready for anything, anticipate everything. However, they must remember that in the end, it is very likely that they will die. Though it can get incredibly frustrating at times, with practice and a great deal of patience, it is possible to get through. The Tank is the worst thing that could happen on Expert, excluding the Witch who can instantly kill a survivor in one hit, but can be avoided. Players can have a small window to fool the lethal Tanks that spawn only depending on the height difference.

Normally, finishing an entire campaign in Expert difficulty may require 2 to 6 hours (in extreme circumstances) of gameplay, and only one or two Survivors can make it to the escape vehicle, though it is quite rare for the final Tank to not incap any and having all Survivors get into the escape vehicle. There are also many unique nuisances in Expert difficulty that you won't see in the others.

Every Special Infected must be taken seriously execpt the Spitter (if the team can avoid the spit in time) and the Boomer (if players can spot him before he/she vomits). Chargers must be taken care of before they start sending Survivors with them into the water, and its best not to split up, as Specials can get rid of a Survivor in ease with their extremely high amount of damage.

The horde
The time between Panic Events in Expert mode is a lot shorter than in the other modes, with around four hordes in three minutes, and the size of the horde is bigger. On top of that, Boomers will constantly be sent at the Survivors (though not as much in Left 4 Dead 2 due to more Special Infected), vomiting and exploding all over them. This makes progressing somewhat difficult as they may be stuck within one area for long periods attempting to deal with the Infected.

Crescendo Events such as the Tower Event on The Parish can cause players great difficulty, due to the large amounts of hordes that come even on Easy diffculty. So if you think about this on Expert, it will take several retries to finish even a small Crescendo Event.

Details
This is a mostly complete list, extracted in part from the exposed cvar interface. See console commands for information on the cvars.
 * The Witch has 1000 health points, can instantly kill a Survivor and is startled in 4 seconds. Witches can only be CR0WND with a headshot (on easier difficulties you can shoot her in other parts of her body and still 'CR0WN' her) from a Combat Shotgun or an Auto Shotgun. If Witches are avoided without being startled the Director ensures to continually spawn them in your path, sometimes spawning three or four per level if the player(s) manage(s) to keep avoiding them.
 * The Tank has 8000 health points, incapacitates Survivors in one hit and deals 150 damage when pounding, making an incaped Survivor dead within 2-4 seconds.
 * Fire inflicts 40 damage per second to the Survivors, which will incap/kill a Survivor in 3 seconds.
 * Friendly fire inflicts full damage to the Survivors. Avoid friendly fire at all costs. One direct shot from a Pump Shotgun or Auto Shotgun will instantly incapacitate a teammate, and cause bots or themselves to waste their very important health kit.
 * Common Infected have 90 health and deal 20 damage per hit from the front, 10 from the back.
 * There is a one-second grace period where the Survivor will not take damage after being inflicted upon. This only affects damage dealt by the Common Infected.
 * Non-headshots deal far less damage to the Common Infected. This case also applies to Smokers, Hunters and obviously Witches and does not apply to Tanks. However, whether it applies to other Special Infected requires confirmation.
 * First aid kits are extremely rare on Expert outside of the safe room and before the finale (except for the Death Toll Finale, and in the Ambulance right outside of Mercy Hospital.); They are instead replaced by Pain pills, which become surprisingly common. However the kits will spawn in the health closets (the ones that hold 1-4 kits/pills if the team has been seriously hurt.
 * The Smoker does 30 damage per hit on ensnared Survivors and does 20 damage per claw.
 * The Hunter does 40 damage per claw, 15 damage every half-second to pounced Survivors and takes 9 seconds to kill a full-health Survivor, or 3 seconds if clawing.
 * The Charger does 20 damage in a punch, 20 damage when its target reaches a wall, and 15 damage per slam.
 * The Jockey does 12 damage per hit while riding a Survivor and 20 damage per claw.
 * The Spitter's spit acid's average damage is 68, and the body acid's average damage is 69 and it does 20 damage per hit.
 * At close range, the Tank is extremely good at cutting off and closing in on Survivors who try and turn, even a little (by turning it any movement other than running directly away from him). The only way to outrun a Tank on Expert at close range is to continue to run directly away from him. The same goes for his rock-throwing attack, it is absolutely unavoidable if you are not paying attention. The best way to avoid his rock throw attack is to continue to move in one direction while he prepares to throw the rock, then when he throws, it change directions quickly. Or simply remain still and move at the right time. Another good strategy is to simply place a solid object like a wall or car between yourself and the incoming rock.
 * The Boomer appears to have noticeably increased range with his vomit attack and it does 20 damage per claw.
 * Enemy reactions are much faster; for example, Smokers will often pause for several seconds on easier difficulties before snaring you with his tongue; however, on Expert, they won't hesitate to attack again.
 * The most threatening Special Infected will almost always spawn on the path(s) you must take, namely the Tank and the Witch. However, observant players might notice a roaring, muscled or hunched, crying mass in the distance, and closed captions can help predict enemy locations, which can help a player get the drop on the Special Infected.
 * Players are penalized more severely if they wander off course, back-throttle, or stagnate at a location, causing zombies to spawn on the paths which you have already taken.
 * Average chances of Survival: 4 Survivors = 40%, 3 = 30%, 2 = 20%, 1 = 10/0%.

Important Tips

 * Play with skilled and cooperative teammates and use microphones.
 * Establishing a good rapport with your teammates helps to foster a good relationship and enforce loyalty, reducing the chances of players quitting abruptly (rage-quitting) and disrupting gameplay.
 * It is advantageous to test-run with a few friends on a map with a slightly lower level of difficulty so as to gain the opportunity to learn when and what to anticipate from your teammates on the intuitive level, as well as gauge their pace, in preparation for Realism settings.
 * When playing on higher levels of difficulty, especially on Expert or Realism settings, it is advised to have good ping (green or yellow bar) while making sure that whoever hosts the game has a good system, as not having a good system can result in lag. Even the slightest delay as caused by laggy framerates or a stuttering connection will mean the difference between being instantly killed by a Witch, or managing to evade her within a split second.
 * Panic events are extremely dangerous on Expert difficulty, make sure you find some corners with only one or two"Attack dierctions". Since you lose 10 or 20 health points after each hit, you may get incapacitated in a few seconds if you are not careful.
 * It is better to play with three human players than Survivor Bots; not only can the game not continue if all human players are dead even if some Bots are still alive, but also the human players are more cooperative and can pick up Defibrillator units (which are important in this difficulty). Survivor bots also do not know how to pick up and utilize tertiary weapons such as pipe bombs.
 * While survivor bots may seem inferior to human players in most ways, there are actually very unique advantages to having one or two bots around, especially for survival mode. For instance, Survivor bots can shoot right through you to kill the infected behind you, while not inflicting any friendly fire damage at all. They also seem to possess a "third eye," which allows them to know if the infected is approaching them from blind spots such as from behind or on top. Finally, bots have an uncanny ability to score accurate hits on the infected, making them invaluable assets against long-range threats like Smokers, Spitters and Boomers. This accuracy combined with their lack of a blind spot makes them ideal sentinels against the Infected without fear of friendly fire.
 * It is suggested that the first player play as Zoey or Ellis, as they use sniping rifles when controlled as Survivor Bots. The Sniper Rifle and Hunting Rifle don't work very well in this difficulty, since Special Infected often attack immediately on sight, and Survivors won't have time to stop and snipe.
 * One Survivor making it into the safe room is enough to advance to the next chapter. Any Survivors who didn't get in the safe room will respawn in the beginning of the next chapter with 50 permanent health, which is usually higher than the health before they get into the safe room; the only downside for this is all items will be lost.
 * However, this can be used to the Survivor's benefit. Simply kill the Survivors that make it into the safe room with less than 50 health, and then close the door to advance to the next chapter. All items will still be spawned inside the safe room, and the dead Survivors will respawn with 50 permanent health (and carry over health kits and other items they had before being killed). However, if not given permission by the other players (if any), you can get voted out of a game for team-killing.
 * Use items for temporary health as much as you can, save your health kits on those Survivors who would die from being hit once. (Recommended: Use pain pills or adrenaline shots when you're at 50 health or below.
 * Use pain pills even if you have less than 30 health or 29 temporary health in order to save health kits.
 * Playing online, it's not as smart to use your first aid kit when you have around 30-39 permanent health.
 * Stay away from open areas, use your Pipe bombs and Bile bombs if you have to fight hordes in open areas. The open areas are only good when you are fighting Special infected such as Tanks.
 * Pick up Defibrillators and Ammo upgrade packs after you used your kit. Incendiary ammunition is good for Tanks while Explosive ammunition is good for Witches and other Special Infected. Deploy them with your teammates before you fight with them. However, be careful with the explosive rounds, as they will damage you and your teammates a lot if it hits very close to you.
 * Attaching Explosive ammunition to a rapid-fire weapon such as a Submachine Gun or Assault Rifle is a good idea as it often takes just one or two bullets to stumble a Special Infected. This is extremely useful especially against the Charger that cannot be shoved off a Survivor as one Explosive bullet can stumble the Charger before he can start pummelling his captive. However, be warned that this causes friendly fire, and this can be deleterious on Expert settings.
 * Turning on captions help you a lot! Go to Options>Audio and turn on captions. It will show "[Jockey Alert]" or even "[Tank Growls]" which can let you prepare by searching for molotovs, gas cans, or propane tanks.
 * Be careful when the rescue vehicle arrives; Survivors often rush to the vehicle rather than risking their lives to save their teammates.
 * Think before picking up a superweapon such as a Grenade Launcher or a Chainsaw. In spite of their vast firepower, the need to frequently hold back due to the avoidance of friendly fire inhibits their utility and forces you to stick to melee or your secondary weapon much of the time.
 * Do not expect lines to be drawn between who should be responsible for looking out for each other, or who should be on the offensive. Expect to take up all different roles of gameplay in almost all situations.
 * While it is possible for a gung-ho player to rush ahead of his or her teammates to complete the campaign as soon as possible, this can sometimes be ill-advised on higher levels of difficulty, where temporarily separation entails permanent segregation. It also yields a bad impression to certain players.
 * Sometimes it is not a good idea to go back and pick up previous supplies, especially on Expert or Realism Settings due to the huge amount of damage from Common Infected.
 * Careful when wielding shotguns or auto-shotguns as they incapacitate teammates in a single direct hit. Use automatic weapons if you are not sure of your aim or you have a slow computer.
 * Remember, building up good reputation among other players is more important than beating the game, especially when playing for the achievement.
 * Do not throw a gas can/propane tank/oxygen tank/fireworks crate in front of an ally you are behind. They might shoot it, which leads to havoc of running around, finding safety, and letting Special Infected having an easier time to pick them off.
 * Also, have a good tolerence to other players' bad moves, even those who cause the whole team to restart. Both humans and Survivor Bots make mistakes, but humans can learn from their mistakes, and you can keep in touch with them anytime.
 * It is always recommended to close any doors behind you before moving on here, this will delay the time for the hordes to come and get you because they have to break down the doors.
 * Melee weapons are much more better then pistols since you can shove off groups of Infected surrounding you, since you kill some Infected and stun others off.

Realism
Realism is meant for people who play frequently together online. It can be activated on any difficulty level and changes gameplay to become, like the name suggests, more realistic, or in the developers' words, "like in the movies." Realism can't be accessed in Left 4 Dead as it is an exclusive mode for Left 4 Dead 2. Playing on Realism at higher levels of difficulty like Expert is ill-advised until you have garnered sufficient practice to grasp an intuitive feel and instinctive anticipation of other players' behaviors. Thus, it is recommended that players should familiarize with each other before attempting Realism.

Details

 * Only headshots are instant kills, and attacks on the torso or limbs cause much less damage than they would normally do. Direct hits with the Grenade Launcher, Magnum, and M60 are still one hit kills.
 * Realism removes the outlines that normally appear around teammates when there is an obstruction between one another. Similarly, outlines around weapons and items also do not appear unless close enough to grab them (items that would normally have blue outlines have no outlines, making it harder to notice a lesser Tier weapon as well). Outlines around Special Infected, such as a Hunter that pounced a Survivor, are also removed. This makes it particularly difficult for players to keep track of each other, and friendly fire will be much more difficult to avoid.
 * Only white and orange outlines remain.
 * White outlines are around items that are within reach, as well as Gas cans that have not been picked up in The Atrium and The Port.
 * Orange outlines are around Gas cans that have being picked up and dropped during the The Atrium and The Port
 * You do not respawn in closets, so if you die, you must either wait until the next chapter or be revived via a Defibrillator.
 * Witches will kill any Survivor they hit, regardless of the difficulty (not on easy); hence, The Sugar Mill requires extreme caution on Realism.
 * Witches cannot be cr0wned with any type of shotgun, forcing the use of ammo upgrades or everyone shooting the Witch at once.

Some possible strategies to cope with the increased difficulty is to have at least one player carrying a Defibrillator along, and consider killing all members below 50 health in the safe room so that they will respawn with 50 health in the next map. Although, keep in mind that sidearms may be lost (i.e. Magnums are relatively rare and are only found a few times in one campaign) or possible misunderstanding of team-killing by other players. It is recommended to do this only in safe-room, as the "main" Survivor can be incapacitated after team-killing. Players might also find it useful to bind their keys to vocalize commands or use microphones to inform each other if they are in trouble.