The Boomer

The Boomer is an extremely bloated Special Infected, which seems to be filled with an odorous slime. They are able to vomit bile, and if it hits the Survivors, it covers them, temporarily blinding them and attracting the horde. When the Boomer dies, it explodes and discharges large amounts of gore and viscera, which will also attract the Common Infected if an unfortunate Survivor is caught in the blast. In addition, both Infected and Survivors are knocked over by it, this could be used to get a Hunter off of a pinned survivor who is stuck on a ledge out of range.

Boomers are relatively delicate, as far as the Special Infected go: it is only able to take a very small number of bullets before it bursts. If Boomers drop from a particularly high place, such as from a roof top into a street, they "pop", which is an effective means of ambushing Survivors. However, the Boomer does not burst when falling from tall heights in Versus.

In Versus mode, players are able to play as Boomers. Their ability to vomit is gauged by a somewhat slow-filling meter, which was probably employed to balance gameplay. Though the Boomer is clearly not one for combat, its melee attack can deal double the damage of a regular Infected, much like the other boss Infected. It's recommended to begin hitting Survivors if you've already vomited on one or more. If they aren't careful, it's possible that they will cause the player to burst, covering any clean Survivors left. Only one Boomer is allowed while on the Infected team in Versus.

Extent of Mutations
The Boomer has been heavily mutated by a strain of the infection. It has become extremely bloated to the point that a slight rupture in its flesh will cause it to explode, sending the bile, blood, and other contents of its body everywhere. It is believed his bloating is caused by the decay of body tissues, creating a build up of gases within his body (which explains the small loop of intestine sticking out where its belly button would be). Massive growths have appeared along its skin, indicating that it may still be expanding. It has developed the ability to project its vomit great distances, and its bile has developed the unique characteristic of attracting hordes of Infected, possibly through smell. Though its mentality is similar to that of the typical Infected, it is not distracted by such things as the flashing lights of pipe bombs, nor is it apparently attracted by things such as car alarms going off. This could indicate that it has some form of intelligence, and is focused on keeping the Common Infected concentrated on potential victims.

Survivors
General:
 * Listen for the heavy breathing, gagging and burping sounds.
 * Keep an eye out for defensive positions, that way you'll be ready if the Boomer blinds you.
 * Avoid being too bunched up with the other Survivors. There's no sense in all four of you getting puked on at once.
 * On expert, finding and killing the Boomer before he can vomit or explode is incredibly crucial as the Boomer bile attracts the horde, which on expert difficulty is much more dangerous than any other difficulty.
 * On expert, Boomers will often claw Survivors they have vomited on, dealing even more damage.

Before bile spit or Boomer death explosion: After being vomited on:
 * If the Boomer is too close, it's a good idea to hit it with your melee attack (which will push it back to some degree) then back away to a safe distance before shooting or you'll get covered in bile, even though you weren't vomited on.
 * CAUTION: You can only safely push a Boomer back about three to four times,if the Boomer had full health.)
 * Only if the Boomer is sufficiently far away from both yourself AND all other Survivors should you shoot it.
 * Hiding behind something or running away when it's vomiting makes it harmless for some time and may even force it to rush at you clawing.
 * Know that a massive horde will immediately descend upon you (and any others who have been slimed).
 * Hold your fire! Since you can't see, friendly fire is the most likely result. You need to:
 * Run to a defensive position (corners and doorways are particularly good).
 * Crouch! Other Survivors will be able to shoot over you safely if you do so.
 * Use melee continuously to survive the incoming zombie horde.
 * Once your sight starts to clear, mop up with your weapon.

Helping a slimed Survivor:
 * Stay close to the stricken Survivor.
 * Help melee the horde back or shoot if the blinded Survivor stays safely crouched.
 * Help the stricken player to safely get to a corner or high ground to protect him from the Horde.
 * Throw a pipe bomb to distract the horde.

Infected
While it may be difficult to be completely stealthy as a Boomer, try to hide in shadows, around corners, or within hordes of Common Infected. To ensure that you slime the Survivors, vomit on them at close range, then run up to attack them so that they will be more likely to shoot you. You will explode and spread the bile within a certain radius around you. Once you cover a Survivor in slime by either of the two ways, the Survivor(s) will appear with a bright purple outline replacing the original one indicating health status.

Your best teammates are the Special Infected.


 * First and foremost, the Boomer can only spawn one wave of Infected per Survivor at a time. Vomiting on a Survivor and then exploding on the same Survivor will not generate additional Infected despite getting one person "twice". To generate additional Infected, you must either explode on a "non-purple" Survivor, or wait until all Survivors are no longer surrounded by a purple aura with bile and then strike again. Therefore; he who pukes and runs away, lives to puke another day. The best tactic for this is the "ninja-boom". If it is possible for you to run away after you vomit on the players then run to hide. Hide farther down the level and wait for your vomit to recharge. Once it has, initiate the same attack - vomit and run.
 * Boomers are large, making them easy targets. If you manage to slime a group of Survivors in a place where running away is going to be difficult, and finding cover will take some time, don't be afraid of running in and clawing them, as this will deal extra damage. While in the middle of the chaos, it can also confuse the Survivors from which direction the horde are actually attacking them from. Also, if they're not careful, they can shoot you once they're clean, resulting in being covered again.
 * However, often it is more effective to back off and find a place to sit and let your bile recharge, as some horde attacks can take up the full extent of your recharge time. If you're lucky and have good positioning (ideally sitting in a window above the group before they emerge from a doorway below), you can score multiple horde attacks by raining bile down upon them each time they emerge after the horde, causing them to have to huddle back inside, depleting their health, and giving your teammates ample time to respawn and deal damage.
 * Since the Boomer has a very distinctive gurgling noise, choosing a good spawn-time is crucial. Do not spawn when the Survivors are far away and attempt to rush them, especially in the open. Even if you are hiding, the Boomer's gurgling will alert the Survivors of your presence. If you spawn inside a room only a second before the Survivors pass the room's door (make sure the door is open/has a hole in it), you can do a spawn-vomit succession without giving them time to react. If you don't manage to cover all of the Survivors, you can either run at them clawing, hoping to explode in their face, or escape and return later.
 * The Hunter can pounce on the Survivor covered in bile to add terrible confusion to the mess; the bile will summon the Infected, who will create a meat shield for the Hunter. By the time the other Survivors are able to get to their pinned friend, it may be too late.
 * On the other hand, a Boomer can vomit on a Survivor who has been pinned by a Hunter to call the horde, then hide from the rescue party or stay and attack, heading to the clean Survivors to cover them when they get close to melee your labour away.
 * If you are playing a Hunter and a Boomer vomits/explodes on the Survivors, they will often huddle up in a corner "spamming melee". This has upset a lot of Hunters who cannot pounce through this wall of melee. The March 25th update added a melee "fatigue" of sorts in Versus mode, but it is not reliable to plan an attack. However, if you, the Hunter, pounce into the top of that corner instead (pounce over the Survivors heads), you will land on a Survivor and his teammates' melee will not hit you. As they are blind, they will have a hard time finding you inside their huddle. This can, in teams who do not communicate well, cause death to a player. Since the Survivors are blind, 2 or 3 Hunters can perform this plunge into the walls top corner without being afraid of getting shot.
 * The Smoker can constrict a Survivor who has been covered, effectively creating a piñata for the Common Infected. This is a very effective way to incapacitate a Survivor quickly.
 * The Tank can distract those who are not covered in bile, cutting them off from helping their friend. It can also incapacitate a covered Survivor, rendering them nearly helpless against the zombie onslaught all over them and adding the chance that they'll bleed to death.
 * During Tank attacks, take the opportunity to vomit on the Survivors. This is a particularly helpful and effective action, as the horde sticks Survivors in place, and forces them to deal not only with a Tank, but dozens of zombies. Often getting a well-placed vomit attack in during a Tank attack spells death for the Survivor team.
 * While a Survivor is covered in bile, they cannot see the outlines of the other Survivors (or that of the Hunter after a pounce). While another Infected is attacking a straggler, take the opportunity to cover the other Survivors with bile. While this may cause the Horde to spread out, it is important to impair the other Survivors' vision so that it becomes more difficult for them to work together and to aim.
 * The Boomer can hide next to cars with alarms and Witches to trick Survivors into shooting them. The nearby explosion will cause the alarm to be triggered or the Witch to be startled (if shot). However, after the new update, a dying Boomer's explosion will no longer cause the Witch to startle.
 * Waiting behind doorways and in small rooms will guarantee at least one successful slime.
 * If you are stuck outdoors, hide in a place where the Survivors cannot see you and wait for them to be distracted by something else, then run in and vomit.
 * The Boomer is the only Infected class as which you should almost always attack an incapacitated Survivor. If you can, try to vomit on prone Survivors so that the Horde comes for them. This makes them almost impossible to rescue as every zombie hit interrupts a rescue attempt.
 * Vomit on Survivors pinned by Hunters or Smokers, unless the Survivors not pinned have low health, at which time it would be best to try to get them incapacitated.
 * Hunters are a great distraction and can let you close the distance without being noticed.
 * In areas with alleyways or other high roofs, jump down into the middle of the Survivors. You'll either be shot in surprise or get an excellent chance to vomit on all four Survivors in quick succession.
 * You can also stand on rooftops and vomit down on the Survivors. Gravity will help the bile to reach them even from great heights. If you explode in mid-air, your innards will rain down on Survivors as well (March 11th update).
 * Don't hide right next to thin walls. Not only can Survivors shoot through most of them, your ample mass may stick through (due to vector errors), giving them a free kill. The slime from the aftermath will, unfortunately, be unable to get through. Try to hide slightly off the wall to minimize cheap kills.
 * In relation to cheap kills, let a Hunter melee you four times before you charge a Survivor. If the Survivors then attempt to melee you, you will boom on them immediately.
 * When you pop, you knock everyone back. This includes, but is not limited to, Survivors, Common Infected, Hunters who have pounced on Survivors, Smokers constricting Survivors, Tanks and even the startled Witch, but for a short time. The good news is that you can knock Survivors off of high ledges; the bad news is that you can do the same thing to Tanks. Take care of where and when you explode!
 * Before the March 25th update, Boomer vomit could be blocked by other Infected. With the update installed, the Boomer's vomit can pass through them and hit the desired target successfully.

Boomer Tips And Tricks Gameplay
Here a video with a lot of helpful information and tactics for the Boomer.

http://www.youtube.com/watch?v=rqc6p5KU06E&feature=channel

Survivors
Blind Luck

You or another Survivor take no damage after being vomited on by a Boomer.

- This achievement requires team work, and a good amount of it too. The best way to do it is to be vomited on in a narrow hallway. Have your team mates surround you, acting as road blocks and protecting you from any incoming Infected. In the rare areas around maps where the Navigation prevents hordes from spawning (the Elevator in No Mercy 4) you may also not encounter a single Infected.

Clean Kill

Shove a Boomer and then kill him without him splashing on anyone.

- An easy way to do this is against an AI Boomer in the No Mercy Apartments. Make sure you're in the room next to the Boomer. Run into the room so that he sees you, then run back out. He'll still vomit even after you're gone, leaving him with a cooldown. Now you can run up to him, punch him without fear of him vomiting, then back up and shoot him. If you see a Boomer (player or AI) out in the open and it is shot (even without shoving) the achievement is still obtained.

Stomach Upset

Survive an entire campaign without any Survivors getting covered in Boomer bile.

- Quite tricky, but can be obtained with increased vigilance for the Boomer sounds and a quick melee. It is almost impossible to achieve this in versus mode as player controlled Boomers are often excellent ambushers, unlike AI.

Outbreak (Only For PC)

Catch a rare strain of infection, then pass it on to someone else.


 * The "Outbreak" achievement is unlocked by being vomited on by a player who already has that achievement.

Infected
Barf Bagged

Cover four Survivors with Boomer bile at once.


 * Can be done quite easily, but usually happens on accident. If the Survivors are grouped up in a corner,(on a finale for instance) this is a perfect chance.

Trivia

 * In the developer's commentary, it's mentioned that originally the Boomer's explosion simply caused damage to nearby players, and that the horde-attracting abilities were assigned to the Screamer, another Special Infected that was cut from the final game. When the Screamer was dropped, the screams that formerly attracted the Horde were changed to the Boomer's bile.
 * The Boomer was the first Special Infected ever conceived.
 * The Boomer is the only Infected that does not appear in the intro movie alive, although Bill is seen to be analysing Boomer bile at the beginning. The recognizable "blown up" corpse of this Boomer can be seen to the left of the screen, against the wall.[[Image:Boomer corpse copy.jpg|thumb|200px|right|The Boomer corpse in the Intro Scene.]]
 * Bill gets a small sample of the bile on his fingertips and wipes it off on Francis, however this wasn't enough to attract a Horde until later.
 * A well placed head-shot will cause a Boomer to die without exploding, but this is rare.
 * It's also possible for a Boomer to die without exploding if he is sprayed by a damaged fire extinguisher, in the hospital hallways for instance.
 * Oddly enough, even when killing a Boomer by shooting it in the foot, the Boomer still explodes, even though its upper body is the part that explodes. Possibly do to a quick reaction in the Boomer's nervous system.
 * When the Boomer explodes, the upper part of its head is launched upwards, falling a few seconds later.
 * When the Boomer explodes or vomits, the bile does not appear on the ground. This fits in, because otherwise the Horde would always be attracted to the site of a Boomer death. As it stands, the Horde is only summoned when the bile comes into contact with a Survivor. It is possible that the Survivor's natural pheromones combined with the bile produces a scent that is strong enough to attract a horde of large size.
 * The Boomer has been considered the hardest Infected to play as, due to how tactical one must occasionally be, all while trying to stay alive with only 50 health.
 * Strangely, the Boomer seems to vomit all the time, but the horde are only drawn to it when the Boomer vomits on a Survivor.
 * When the Boomer explodes, the arms disappear, possibly because they were blown to bits. If there was that much force in a Boomer's explosion, it would more than likely do at least some damage to the Survivors.
 * Judging by the weight and immensity of the Boomer, its body should have been able to restrict the Boomer from jumping higher than a foot or climb ladders since the stress/tension would cause the Boomers' internals to strain and explode. However, for gameplay purposes, Boomers are given as much freedom as their Common Infected counterparts.
 * When examining the Boomer's navel, players can seen the Boomer's intestines protruding from it.
 * The Boomer can be killed by close combat weapons instantly in Left 4 Dead 2, although this is probably a very bad idea as the killer would get covered in Boomer bile from the explosion.
 * Skilled Boomers that can puke on Survivors and escape without being seen are often referred to as 'Ninja Boomers' by Left 4 Dead players.
 * Some Left 4 Dead players refer to being vomited on as being "Boomed" or "slimed" on.
 * A Boomer was apparently hit by a police car at the beginning of Death Toll as a pair of Boomer legs are lying against the police car. This might be the reason for the large amount of dead bodies lying around the same area as if a horde were summoned.
 * As with the other playable Special Infected, human-controlled Infected can revive incapacitated Survivors through use of the same button that a Survivor would use.
 * A Boomer is seen hanging by his limbs in the Left 4 Dead 2 campaign, Swamp Fever. Whether this was done prior to becoming a full fledged Infected or not is debatable.
 * Oddly, the Boomer is, along with the Witch, the only Special Infected not wearing shoes.
 * The Boomer may have been obese prior to infection instead of becoming obese because of it.