The Mouth is a Special Infected with a very grotesque appearance that can call an army of undead to attack you. It would be wise to make them a priority target as their scream attracts a rather large horde of skinless zombies. They can be very devestating if you do not have any weapons that can easily clear crowds, as the Mouths themselves are not the real danger, but rather the mob they send at you. The Mouth can be taken down with a single charged rifle shot on Brain Dead difficulty. The mouth can also be killed relatively easily with melee attacks, the melee attack also stuns it for a second, allowing repeated attacks. Be warned though that it's melee attack isn't anything to ignore either, if given the chance it can easily kill a careless player. The Mouth usually drops med kits.
In higher difficulty levels close range shotgun blasts work well. To kill the mobs, use the SMG, the Blade Cannon, the Zapper, or the Flamethrower.