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, it's contained in a vial, and based on the achievement Septic Tank, the Infected summoned by the bile will attack whatever the bile is covering.

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. This is evident by the achievement requiring one to cover a Tank in the bile, which would cause other Infected to attack it.

The HUD symbol for the Bile bomb is a biohazard symbol.