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 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 is recommended for beginners and those whp want to try new strategies out. This mode is also used when grabbing achievements as the Infected are less dangerous compared to other difficulties.

Details

 * Common Infected receive &times; 2 damage to the body 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 Tank has 3000 health points, does 24 damage per hit and 75 when pounding.
 * The Witch receives &times; 2 damage to the body and is startled in 8 seconds. She does 12 damage on an incapacitated Survivor every half a second (every 5 seconds in L4D2).
 * 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 counted on a 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. However this only happens if they have Explosive Ammo on their weapon.
 * Pipe Bombs, oxygen tanks, propane tanks, and Grenade Launchers 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 the mentioned explosive objects and damage themselves.
 * In The Sewer, Survivors can incapacitate or kill themselves by shooting the gas station.
 * The Smoker deals 10 damage per hit on ensnared Survivors and has a 5 seconds reaction time before attempting to ensnare one.
 * 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 32, the body acid's average damage is 32 and does 1 damage per hit.
 * The Boomer will not vomit as soon as it sees you, possibly around a 5-7 second reaction time before vomiting.
 * Special Infected such as the Smoker take longer to aim before attacking, and Special Infected usually don't sneak up on you, rather than specific Smokers.

Normal
Normal mode is fairly balanced, as far as the difficulties go. It is 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

 * Common Infected receive &times; 1 damage to the body 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 Tank has 4000 health points, deals 24 damage per hit and 75 (24 for Versus) damage when pounding.
 * The Witch receives &times; 1 damage to the body, and is startled in 5 seconds.
 * Friendly fire inflicts 10% of the weapon's damage to the Survivors.
 * The Smoker does 10 damage per hit on ensnared Survivors, 2 damage per hit (and 3 while dragging in Versus). And has 2.5 seconds reaction time before ensnaring a Survivor.
 * 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, the body acid's average damage is 69 and does 2 damage per hit.
 * The Boomer have 3 seconds reaction time before vomiting.
 * AI special Infected such as the Smoker take longer to aim before attacking than in Advanced and Expert difficulty, but take a shorter time than Easy difficulty.

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 a single player is not careful, then the entire team 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 Director feels generous. Tanks pose a serious problem as they gain double amount of health points compared to Normal, needing Survivors to adopt more effective tactics for dealing with it. Common Infected are also more dangerous, but still manageable with teamwork. Other Special Infected, excluding the Witch and Tank, will hide better, and in the Smoker and Boomer's cases, use their abilities immediately rather than delaying them.

Players are recommended to start using weapons that have high ammunition, a scope, a Magnum pistol or at least a fast melee weapon when using this difficulty. Common Infected can no longer be killed in one shot with a Magnum pistol at a fair distance, the Special Infected begin to play smarter and have instant reaction times, and damage is increased from all sources. Headshots are highly advised against Commons and Specials to deal more damage against them. An average Advanced playthrough will usually take anywhere from 50 minutes to 1 hour depending on playstyle.

Details

 * Common Infected receive &times; 0.75 damage to the body 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.
 * The Tank has 8000 health points, does 33 damage per hit and 100 when pounding.
 * The Witch receives &times; 0.75 damage to the body and is startled in 5 seconds. She also does 68 damage to an incapacitated player every half of second (every 5 seconds in L4D2), making it dangerous if you have no assistance and she has high health.
 * Molotovs and other explosives deal more damage onto allies than before. Be careful where you throw them!
 * Friendly fire inflicts 50% of the weapon's damage to the Survivors. Be careful when firing at close quarters.
 * 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, 15 damage per slam and 2 damage if Charger missed his target, but stumbled him.
 * The Jockey does 8 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.
 * 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 Advanced and Expert they won't hesistate to attack again.

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 players 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 players 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.

Special Infected are also much more deadly on Expert; although they retain the same reaction times as from the Advanced difficulty, their damages are greatly increased, especially their clawing attacks. They will have a much more smarter AI, such as Boomers falling at high elevation to explode on nearby Survivors or Hunters clawing rather then pouncing on Survivors. This makes it very difficult should they accompany a horde as the players are most likely to be downed from either them or the Commons. You will want to be careful to avoid friendly fire; a player hit by a full shotgun blast by another player will be immediately incapacitated. The Magnum Pistol will also be unable to kill Common Infected in one shot, unless it is aimed at the head.

Damage by the players are reduced greatly to the point where it almost deals no damage at all; on Expert Realism point-blank shotgun blasts to the Witch will only deal a mere 200 damage to her. You may want to have a variety of weapons to deal with both long and close-range skirmishes. Melee weapons retain their damage rates to the Infected, but because they hit much more harder pistols may be recommended instead. Experienced players should be listening to the sounds of the Infected and have captions on; this will give them more awareness against incoming hordes and if Special Infected have spotted the players.

Expert can be done by yourself with the aid of the bots, but it is usually recommended to play in a full human team as bots cannot carry items and do not use items as they should be used on Expert.

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.

Gauntlet Crescendo Events such as the Tower Event on The Parish can pose players great difficulty, due to the endless amounts of Common Infected. Pipe bombs are often one of the only options to disperse the horde long enough to reach safety.

Details

 * Common Infected receive &times; 0.5 damage to the body 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.
 * The Witch receives &times; 0.5 damage to the body, can instantly kill a Survivor and is startled in 4 seconds.
 * The Tank has 8000 health points, incapacitates Survivors in one hit and deals 150 damage when pounding (killing an incapacitated Survivor in two hits).
 * Fire inflicts 40 damage per second to the Survivors. Be careful with Molotov throwing and gas can placement.
 * 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.
 * First aid kits are extremely rare on Expert outside of the safe room, before the finale, and at other designated guaranteed first aid kit spawn areas (such as the area to the left of the alley after traversing Bienville Park in The Parish and in the ambulance right outside of Mercy Hospital); They are instead replaced by pain pills, which become surprisingly common. However, health packs may spawn in the health closets if the team has been seriously hurt and possess limited or no forms of healing.
 * The Smoker does 30 damage per hit on ensnared Survivors, does 20 damage per claw and takes 8 seconds to kill a full-health Survivor.
 * 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 incapacitate them in 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 in 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, 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 does 20 damage per claw.
 * 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 Infected to spawn on the paths which you have already taken.

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. Survivor silhouettes and item glows are disabled, closet respawns are disabled, and Common Infected take much less damage to bodyhits, making melee weapons or headshots essential.

Details

 * Common Infected receive &times; 0.5 damage to the body on Easy difficulty, &times; 0.37 on Normal, &times; 0.25 on Hard & Expert. They are still killed from one shot to the head from any weapon, and from any damage from Magnum, M60, and weapons loaded with Explosive Ammo.
 * The Witch receives &times; 0.5 damage to the body on Easy difficulty, &times; 0.37 on Normal, &times; 0.25 on Hard & Expert. She still receives &times; 1 damage to the head, but can no longer be CR0WNED with shotguns forcing the use of other tactics such as using special ammo, burning and running away, and others. See the Witch's page on killing her.
 * Witches will kill any Survivor they hit (except on Easy difficulty); hence, The Sugar Mill requires extreme caution on Realism. If a Witch incapacitates you on Easy, after revival, a small chunk of your health is taken away.
 * 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. Given the above circumstances players are expected to rely more on sound to locate one another when they get separated.
 * 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 with a Defibrillator.

Some possible strategies to cope with the increased difficulty is to have at least one player carrying a Defibrillator along, and consider killing anyone below 50 permanent health in the safe room before transition to next map for them to respawn with 50 health in the next map. Although, keep in mind that secondary weapons will 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 for the dropped equipment to carry over. Players will also find it useful to use vocalization commands (Z and X by default on PC) or use microphones to inform each other when they are in trouble.

Realism doesn't affect Bot's behavior much, if at all. This is because the Bots will follow you at all times, so if one player is struck by a Special Infected, they will come to your rescue. This way, players won't need to talk with each other as much when being attacked by Special Infected, and their accuracy against Smokers and other Specials is superb to that of a player.