Screamer

The Screamer was a Special Infected in the early stages of game development, preceding the Witch and the Boomer's puking attack. The idea of the Screamer was for it to lurk, much like the Witch, in different areas, paying no attention to its surroundings until coming in contact with a Survivor. At this point, it would sound a warning, like the Witch. If agitated enough, it would not attack, as it is bound in a straight jacket, but run off to hide. Once hidden, it would let out a scream, attracting a horde, like the Boomer's bile. It was therefore important, to kill the Screamer as quickly as possible while it tried to run off.

When the Screamer was still in use in development, all the Boomer did was explode when killed, splash damaging nearby survivors. The Screamer was cut and is not featured in the game because it was hard to spot among Common Infected. The Boomer then got its bile attack and new bile splash death.

In early versions of the intro movie, as Bill and Zoey are checking out the Witch, a scream could be heard before the horde attacks. The scream is a definite one, and doesn't seem to belong to any members of the horde. While this may be coincidence, it could be that this is the scream of the Screamer.