Bile Bomb

The Bile bomb is a new weapon appearing in Left 4 Dead 2. The weapon was first mentioned in the new achievement list here. Not much is currently known about this weapon, except that it contains Boomer bile, which, when used, will call a horde to attack anything the bile hits.

Once 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 The Parish. There is an achievement that requires you to pick up 10 of these containers. It will use the same inventory slot as the pipe bomb and the molotov.

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 biohazard 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, the infected will ignore the survivors so long as the bile is still in effect making for easy targets.