Berserkers. The Berserker zombies can run over 30mph, are extremely strong, and love to torture their victims for as long as the person can take before biting them. And they do all that while grinning and chuckling. And they sometimes move in groups of ten to twenty.
Basilisk zombies. Zombies that eat you alive are bad. Zombies that paralyze you by looking at you, whereupon you can't fight, run, or scream while the thing shambles over to eat you is... bad.
Mrs. Walters lost her son while he was out on a forage with the main characters. She committed suicide, but not via headshot. Meaning there is now, officially, an outbreak on the ARC.
The very concept of a Mercy is actually probably more terrifying than the others, imagine it, you are walking throught the wastes and all of a sudden this zombie just starts following you not chasing or trying to eat you, just simply following you, the idea being that Mercys follow or try and find the sick and dying, and do not stop until you stop from exhaustion or sickness, then it comes to you, and while, yes, it fends off other, more vicious zombies, that is only so that it can kill you itself, at the very least, this zombie kils you the quickest, through a bite to a vital artery, and then as you lay dying, it comforts you, making soothing sounds or stroking your face, sounds nice in comparison, except that it, in context, only spared you from other zombies to kill you and make sure it's victim rises again as a zombie.
Facial Horror goodness. And after.