Bile Bomb

"Give the infected a taste of their own medicine with this CEDA-developed gastrointestinal projectile. Any zombie you soak with this puke grenade will be swarmed by Common Infected."

- Left 4 Dead 2 item description

"Is that a bottle of puke?"

- Nick

The Bile bomb 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. In many respects it can be considered a tier two pipe bomb, having far superior horde attracting effects, but lacking an explosive payload.

If you kill one of the Hazmat Infected, they may drop one of these containers full of bile due to the fact that they were CEDA workers that were studying The Infection. Despite this, they can be found outside of Dead Center. 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. The HUD symbol for the Bile bomb is a bio hazard symbol. 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 call the horde to attack the Tank. 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 SI which have been hit to fight one another as well as the oncoming 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 times on when to use a bile bomb and when not to use it. For example, doing it on a lone infected would be pointless because you will then have to kill more targets after the effect is out. On the other hand, highly dangerous infected (with the exception of the witch) like a tank should be covered from the bile from the bomb to get rid of him.
 * The bile bomb is like a mixture of a pipebomb and a molotov. It has the horde attracting power of a pipebomb 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, it's okay not to hit them, but you could attract them into one area. If it's a Tank, then hitting him directly counts.
 * Obviously, throwing a Bile Bomb makes common infected easy kills, allowing you to conserve ammunition by using your handgun or melee weapon of choice on the distracted infected. This could also make them useful in combination with the Heavy Machine Gun. They will blindly swing at the bile, however, and getting too close will cause them to switch targets (unlike blocking commons when a player has boomer bile, in which case they will ignore you and focus on slimed players).
 * 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 rolling crescendo, a finale when the rescue vehicle arrives, or a gauntlet finale.
 * CEDA agents are far more likely to drop this on easy mode. This is helpful when going for the achievement
 * While it 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 will likely be 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 simple distracting effect using the same programming as the pipe bomb.
 * Be careful when using a Bile Bomb on a Witch. If you startle her, by the time the infected begin to attack, the Witch will probably have incapacitated or killed the thrower and run away, leaving you and your allies with an incapacitated teammate and a horde coming in. The infected and the bile(unlike a molotov) won't slow her down, either. However, if you stay out of range, she will not notice who threw the bomb, and will instead run away crying as the infected chase her down. Plus, when she does incapacitate a survivor, she would just be taken care of like if you or an ally just got incapacitated: Just fire on the witch while she's ripping the survivor apart until she's dead.
 * 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 is 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 Jar 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.

Trivia

 * The concept for the bile bomb was mentioned by a user on the official Steam forums . Credit for the weapon however has not been assigned, as there has been no proof that one single fan came up with the idea, or if Valve coincidentally created it.
 * It is odd how bile does not cover any types of Infected when the Boomer vomits on them or explodes near them, yet when you throw a Bile Bomb at a group of Infected, it seems to spread all over them. This coupled with the fact that the Bile Bomb is a different color then normal bile, probably means CEDA altered it so it only goes on Infected and not Survivors.
 * The bile moves around in the jar as you walk around like the rag and liquid in the molotov.
 * The "blinding" effect of the Boomer bile also affects players as Special Infected.
 * The bile bomb is currently the only throwing weapon that does not look like it will explode on the carrier's hand, unlike the molotov or the pipe bomb.
 * The Boomer bile bomb may indicate that the military are trying to research and create a effective bio-weapon against the Infected. One military application could be to gather all the Infected in one place for execution. Or perhaps, simply, it was to extract more information on the Infected.
 * It seems that CEDA may have came up with the names for the Infected, or at least the Boomer, since the label clearly states "Boomer" Excretion.