Bile Bomb

"Is that a bottle of puke?"

- Nick

"Oh man! A jar is no place for bodily functions."

- Ellis

The Bile bomb (called "vomit jar" by the game when spawning items or creating scripts) is a new weapon appearing in Left 4 Dead 2. The weapon was first mentioned in the new achievement list here. The bile bomb can be found in Left 4 Dead 2 as a throw-able glass canister containing the unmistakable viscous green Boomer bile. When thrown it summons the Infected horde to attack whatever it hits.

Bile Bomb is often found in the first two levels of Dead Center. It may also be dropped by Infected CEDA Workers, though this is rare. The bile bomb will use the same inventory slot as the pipe bomb and the molotov, so you can still only carry one of the three at a time.

The bomb spreads Boomer bile on whatever it hits and the horde summoned by the Survivors will attack whatever is covered in bile, including other Infected. If the bile bomb is used on an empty space, it will summon a horde and all of the local Infected to the place where the bomb landed. Sometimes the Director will even spawn a horde if there are not a lot of Infected, and the Infected will ignore the Survivors so long as the bile is still in effect, making them easy targets. The Common Infected hit by the bomb will begin attacking each other, as well. The bomb can also help in Tank fights as it calls a horde to attack it. The effect of this is only marginal, however, and usually serves to slow the Tank and damage it slightly rather than kill it. Frequently, though, the Tank will be rendered immobile by the common hitting it. This can buy the Survivors a lot of valuable time - just make sure you don't let any commons survive, especially on Expert.

The bomb can also be used to confuse Special Infected as it will make any two such Infected which have been hit to fight one another as well as the oncoming Common Infected. When a bile bomb hits an Infected, Survivors will be able to see the affected Infected's aura. Throwing the bile bomb on a Survivor will not draw a horde to attack; however, the horde will be drawn to the location where the bomb impacted.

Tactics

 * Despite the intention of the bile bomb as a way to get enemies to fight each other, a far better use for the bile bomb is as a distraction during a crescendo event, or as a distraction while quickly running as far forward through the level as possible to find the next defensible location. If you leave common Infected far enough behind, they won't chase you.
 * The bile bomb is a very valuable tool. You should consider carefully when to use a bile bomb and when not to use it. For example, using it on a lone Infected would be pointless because you will then have to kill more targets after the effect is over. On the other hand, highly dangerous Infected, like a Tank (with the exception of the Witch, as you need to be VERY far away when doing so, as not to startle her, making it difficult to hit such a small target), need be covered by the bile from the bomb to get rid of them.
 * Also be warned that if vomited on, the attraction to you is far greater than that of the bile bomb. So if you get covered in bile, don't bother wasting this grenade.
 * The bile bomb is like a mixture of a pipe bomb and a molotov. It has the horde attracting power of a pipe bomb but any Infected caught in the blast will get stuck with the bile, like if you were to be caught on fire with a molotov. If it's a horde, you don't need hit them directly, rather you could attract them into one area. If it's a Tank, then hitting him directly counts.
 * Boomer bile bombs also distract Infected for a longer time than Pipe bombs, making them potentially more useful for situations where the Survivors need to run from one point to another with Infected attacking, such as a Gauntlet Crescendo, a finale when the rescue vehicle arrives, or a gauntlet finale.
 * While it often won't kill a Tank, a bile bomb thrown at a Tank will cause common Infected to slow him down and damage him. If you use this tactic, try to shove away the attacking Infected instead of killing them. The bile bomb gets riskier and riskier with rising difficulty, due to the higher health of both the Tank and the lingering Common Infected after the bile wears off.
 * Bile bombs are much more useful in Hard Rain than in any other campaign, where the Worker Infected ignore pipe bombs, but still chase bile bombs. They will, however, ignore a bile bomb that did not hit any Infected due to the fact that the distracting effect uses the same programming as the pipe bomb, which Worker Infected are programmed to ignore.
 * It is usually more useful to use a Molotov on a Witch than a Bile Bomb. Bile bombs will not slow her down and the Infected that come to attack her will do little to stop her. It is however possible to throw a Bile Bomb from far away and not startle her but this is difficult to do.
 * If you have a specifically powerful target, like a Tank or Witch, it is better use the bile bomb only when a massive number of Infected are already nearby. Otherwise, the Tank/Witch will probably be dead by the time your horde shows up, and then all you have is more targets.
 * The bile bomb is considerably much more valuable than it already is while in versus, where Infected are controlled by players and it confuses them more than if thrown on an Infected bot. Because they're blind and are unexpectedly being attacked by their own team (AKA the Common Infected), they often panic, especially Tank players.
 * In a Versus round. If the ending safe room door is open, throw a bile bomb in there to find lurking player-controlled Infected as well as giving you time to kill them due to the Boomer blinding effects.
 * In the absence of a needed pipe bomb or molotov, one way to use the bile bomb is to throw it on or near a propane tank, gas can, or oxygen tank. Wait for the Infected to swarm it, then shoot the gas can/propane tank/oxygen tank.
 * Finding a bile bomb while having a molotov cocktail or gas can or vice versa can lead to some devastating results if used correctly. By throwing the bile bomb First then quickly throwing the molotov cocktail in the same area you previously threw the bile bomb. The result, all the common Infected horde will run into the flames allowing quick kills and a lot of saved ammo. This is useful for getting to an escape vehicle finales and killing multiple Infected at once. If you use a Gas Can, be sure to place in an area you can see it before igniting it.
 * Similar to the molotov strategy above, it is effective to throw a bile bomb into, or near, the flames of the Hotel on Dead Center. All the Infected in the area, or that are spawned, will run into the flames and instantly die, allowing you to quickly get your bearing to find your way through the flames.
 * If in a high area, such as The Bridge or The Hotel, throw the bile off the side of the ledge. Infected will swarm off the edge and fall to their deaths. Pipe bombs also work with this strategy. This will not credit you with kills though.
 * When throwing a bile bomb to avoid hordes of Infected (like in the Parish bridge run or Dark Carnival gauntlet at the barns) do not shoot any Infected going for the bile as the director will respawn killed Infected immediately. The Infected will gather around the bile emptying the area effectively if left alive.
 * It is wise to preserve bile in levels with Gauntlet Crescendos. Note that in Dark Carnival it only spawns in the first and last map and in the Parish, only in the second map). Teams should plan ahead for these events and have two players carry bile for these occasions.
 * Sometimes it is ill-advised to throw a bile bomb on a Tank, since the bile bomb rarely kills the Tank outright (especially Advanced upwards; even Normal sometimes). However, it does slow it down. But this can be achieved by continuously firing at it. It is better to save the bile bomb for the hordes.
 * The bile bomb can be a good way to deal with a horde when a Wandering Witch is around in close quarters. If a pipe bomb were thrown, the Witch may walk up to it while the bomb explodes, forcing you to fight her. Just be sure not to accidentally hit her when throwing bile.
 * The bile bomb can be used to greatly reduce or eliminate the number of Common and Uncommon Infected that you will encounter in a level, up until the first crescendo or panic event if it exists. If you throw a bile jar into the safe room, close the door before any Infected get inside, and refrain from killing any Infected that attempt to enter the safe room, then a large group of zombies will indefinitely attack the door. Since the number of live Common and Uncommon Infected is limited by the Director, the rest of the level will contain few Infected or none at all. Thus, if this exploit is to be attempted, it is recommended that you kill any computer-controlled Survivors beforehand, or they will prevent the Infected from gathering at the safe room. This exploit only works on levels other than the first or finale of any campaign, and you will still encounter the Special Infected.
 * Bile Bombs are often found in the first two levels of Dead Center. There is always at least one bile bomb in the saferooms of Hard Rain and as well as the Burger Tank. However, as mentioned above bile bombs in Dark Carnival as well as The Parsih are extremely rare. They are often only found in certain sections of the campaign, like the first and the last chapters of Dark Carnival and the second chapter of The Parish.
 * A common misconception (typically of rookie players) is that two bile bombs thrown at two Tanks (such as the two spawned in the finale of Swamp Fever) will cause them to fight each other. This, needless to say, does not work. Do not waste bile bombs for this purpose.
 * If in an area of a map where you can toss the bile bomb outside the map (where there is no nav mesh for the infected bots), the common infected will stand in one place and dance (the attacking animation). This leaves your team the ability to rush far ahead in the map without them attacking until the bile bomb wears off.
 * After being vomited on by a Boomer, the Bile Bomb becomes useless until the bile wears off. The common infected will chase after you rather than the bile on the ground. It is unknown why.
 * The Bile Bomb also works well in conjunction with the Grenade Launcher, especially if the Infected in the area are too scattered to make efficient use of the launcher's blast. The bile will draw the Infected out into the open, and they will pack together, making them an easy target. This strategy can be employed to more easily earn the DISMEMBERMENT PLAN achievement.
 * The aura around Infected covered in Boomer bile will not appear in Realism mode. Hence, the only way to tell if one is covered in Boomer bile is to see the bile dripping off the Infected. Still, it is best to remember that the aura should not be relied on in Realism mode to spot any Infected affected by the Boomer bile effect.