So when a zombie bites you, it's bad. We all know this. Any consequence from a Teeth attack will eventually result in the victim becoming a zombie, no two ways about it. It'll worsen from scene to scene, moving from mild to moderate to severe, until the victim is taken out... by becoming a zombie.
There's only one way to "treat" a Bitten consequence, and that's amputation. It's not the hardest surgery in the world, but man, you gotta do it fast. Regardless of the severity of the consequence, the difficulty to treat it is +6, and doing so gives you a severe consequence appropriate to the amputation. (I recommend Dis-Armed. I use it every chance I get.)
If that first attempt fails, no further attempts can be made. You are doomed. And if that Bitten consequence was already severe, then... see the previous sentence.
This is pretty rough, but a zombie can't actually make a Teeth attack without first invoking an aspect. It doesn't get the usual bonus or reroll for invoking this aspect -- all it gets is the opportunity to bite. We used to call this "invoking for effect." The idea is that it'd create an advantage with Claws, like Grabbed, then bite with its next action. Or the victim might be Surrounded, or maybe that Sprained Ankle slows them down. You get the idea.
- I had this idea that zombies would only roll 2dF. They're reliable, predictable, and experience neither highs nor lows. But I'm uncertain about this one.
- Mobs are important to zombies and their particular brand of horror. Even though they take consequences individually, I'd still mob 'em up whenever possible. So they attack you en masse, but you attack them one-on-one.
- There've been quite a few scenes in The Walking Dead of people just head-stabbing helpless zombies through a chain-link fence. As always, if there's no interesting failure result, don't roll for it; it's just narrative color. "So you're out on fence duty, braining walkers with a crowbar, when all of a sudden...."
- If you need zombies to use a Physique-type overcome action, you could use Body. Or you could use nothing, and rely solely on mob-based gang-up bonuses. This is pretty cool, I think, because it means you can pretty easily keep one zombie at bay with a closed and locked door, but a pile of them will break through, given time.
- Brains. BRAAAIIINS.