in left 4 dead wiki it says that the witch was pregnant when the infection began and her baby died.but somehow the baby gave her some special powers.and if you shoot the witch you will most likely die.also she only attacks the people that shoot her or make a lot of noise.you will know when theres a witch because you will hear crying or if you put your tv on mute and you cant hear you will still know because the team will turn off all there lights and walk quietly