Screamer

The Screamer was a Special Infected in the early stages of game development, preceding the Witch and the Boomer's vomit 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. The Screamer did not have any actual attacks. At this point, it would sound a warning, like the Witch. If agitated enough, it would not attack, as it is bound in a straitjacket, 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.

As stated in the Developer Commentary, when the Screamer was still in use, all the Boomer did was explode when killed, splash damaging nearby Survivors. The Screamer was then cut and is not featured in the game because it was deemed too difficult for Survivors to spot and track him down amongst Common Infected. The Boomer was then given its bile attack and new bile splash death.

Trivia

 * The straitjacket is usually used as a restraint to prevent mental patients from harming themselves and others. Since the Screamer is bound in a straitjacket, it hints that the Screamer was formerly a mental institution patient, or was restrained due to the Infection's crazed symptoms.
 * The Screamer bears a resemblance to the Siren in the game Killing Floor. Both wear straitjackets and both scream.
 * The Jockey has sounds which are probably re-used from The Screamer, as he was also supposed to cackle like a maniac.
 * Judging from Concept art, The Screamer has the same shoes as the Hunter.
 * In a very early Left 4 Dead trailer, the scream that attracts the horde and sounds like the Hunter is the Screamer's voice, but is now used as the Hunters when he leaps.