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 and based on the achievement Septic Tank, looks like the Infected summoned by the bile will attack whatever is covering it.

Once you kill one of the Hazmat Infected, they should drop one of these containers full of bile. There's an achievement to pick up 10 of these containers after killing the Hazmat Infected. It will most likely use the same inventory slot as the pipe bomb and the molotov.

It has been speculated that the bile bomb will have similar functions to the pipe bomb, as Boomer bile calls the horde. The bomb could do the same and the horde summoned by the Survivor will attack wherever or whatever the bomb hits, including other Infected. This is re-enforced by the achievement gained by covering a Tank in such bile, which would probably cause other Infected to attack it.

This weapon may only be used in extreme circumstances to remove or null large threats, such as a Tank or Charger, even if it means mopping up a horde after the bile wears off or the summoned Infected have killed their target. Even worse, if you hit a team-mate with friendly fire, then the devastating effects of the Boomer bile will more than likely be applied to them. It could be that this grenade was designed to be most effective against Special Infected, as a balance of the Common Infected killing pipe bomb and the all-round-effective molotov.