The Witch

From wikipedia

The only mutant infected that will not be available to players. They are passive upon discovery by the players, but react with hostility to flashlight beams, physical contact, and gunfire in the vicinity. It will be able to kill a player in one hit, and has almost as much health as a Tank, making them the most dangerous of the infected. She has the resemblance of average infected, but can be heard crying to give the players a chance to avoid or attack her. If she detects a survivor, she screams wildly and will kill the nearest player. She will then resort to regular attacks. During these attacks she slashes so viciously that the survivors' blood will spatter all over the adjacent area. The witch will not be included on the easy difficulty.