Boomer (Left 4 Dead)

"Well, they've been calling those Boomers. For obvious reasons. Again, it's a Boomer, so don't let it puke on you, or the Common Infected will fight you down."

--Su Ji-Hoon, Puking is Not an Opinion

The Boomer is an extremely bloated Special Infected. Their mutation has caused them to produce immense quantities of bile which they attempt to vomit on the survivors, temporarily blinding them and summoning a small horde. Though fragile and slow, the Boomer can be dangerous to a survivor team whose health or combat abilities are compromised.

The Boomer's role in Left 4 Dead and Left 4 Dead 2 is to slow down, blind and sow general confusion among survivors, giving the hordes of Common Infected of any species openings to mount attacks with higher probabilities than normal to achieve damaging results. The known presence of a Boomer may also cause survivors to spread out a little (to avoid being coated in bile) and this too works to the advantage of Special Infected such as Smokers or Jockeys when mounting a coordinated attack.

In When the Cold Breeze Blows Away, like Crazy Ivans, they are suicide bombers that serve for the Undead Alliance. They also have an ability to puke their vomit at enemies.