Infected: Witch

WITCH: The Witch, a passive female infected, who, when provoked by loud sounds, lights, or proximity of survivors, will attack her provoker. She is able to incapacitate or kill the provoker (depending on the difficulty setting) in one hit. If her victim survives, she will attempt to kill them if not killed herself. She can be completely avoided if the player doesn't wish to provoke her. She is well heard by an annoying and far-reaching cry of distress. To be too close is usually a point-blank concept as players who've walked right by her, directly have not been attacked. -Wikipedia WITCH:

The Witch is a unique boss infected that is only controllable by the AI, the player may not control it.

The Witch will not move until she has become aware of the Survivors. Until this time, she will lay on the ground, moaning. It is possible for the Survivors to sneak by without alerting the Witch, if they move slowly and quietly.

If the Witch is alerted, she will run quickly towards the Survivors. She has a lot of health and is very quick. -Left 4 Dead 411

WITCH: The Witch is a member of the Infected, and the only one initially passive to the Survivors.^[1] She is also the only non-playable Special Infected, deemed to be "too dangerous" by Valve.^[2] She resembles the average female Common Infected, except with some notable mutations. The Witch makes loud and eerie noises which resemble sobbing when in her 'passive' state. The noise can be heard long before the Witch is ever seen, allowing the player a chance to prepare a plan to deal with her. In the night-time she crouches and slumps over, mostly immobile. During the daytime (in Left 4 Dead 2 only), she will sob standing up and will slowly wander, with her elongated fingers up to her face. The Witch, like the Boomer and a few other Special Infected, can cause problems when you shoot her. When a Survivor gets too close to a Witch or damages her without killing her outright she becomes 'startled'. When she is startled, she screams and advances on the Survivor that has startled her with great haste. Once she has reached the Survivor, she will slash at him/her, instantly incapacitating (or actually killing, only on the Expert mode or Realism on any difficulty) him/her. If she is still alive and the Survivor is dead, she will run off sobbing out of sight. The exceptions to this result of damaging her being a close up headshot with a shotgun, known as "cr0wning" her, a well-placed Pipe bomb, or (on Easy difficulty) one hit with the Chainsaw. -Left 4 Dead Wiki

A Witch from "Left 4 Dead".