Oofta
Legend
Yeah? So? I don't see the problem. Do people's heads explode when the game's zombies don't match a movie zombie?
The "zombie eats flesh, and the bitten turn into zombies" is a pretty narrow story space to deal with. It is fine when you specifically want a zombie movie, but otherwise it kind of takes over any world in which you put it. And, if the game's zombies worked that way, the Create Undead spell would be a risk of an extinction level event every single time it was cast, and thus *WAAAAAY* higher than 6th level.
I have thought about running an apocalyptic zombie game where there's an infectious version of zombies slowly taking over the world and I think it would be tough to implement, especially depending on how infectious the undead are. Do they have to get a good chomp in? How are they going to do that against someone in plate? Doe a mere scratch do it? Short campaign, have fun playing zombies vs humans.
If you tweak the mythos a little bit they could be like wights, the victim only turns if killed. Or maybe being reduced to 0 (or some other threshold) means you have a chance to be infected, perhaps treating it something like lycanthropy. So with magic there's a chance to recover but there's not enough magic around to cure everyone. I think the real danger (if following modern zombie tropes) is that anyone that dies becomes a zombie. At a certain point if there's a plague or other significant cause of mortality the problem is not the zombie horde outside the walls, it's the child or your spouse sleeping next to you that die in the middle of the night.
In any case, when I've done anything like this there was a significant source of necromantic energy driving the hordes so it was always a self-contained mini-apocalypse and it was never infectious. But maybe someday as survival horror mini-campaign.