Yeah, at this point in the series we are at least a year into a society with no modern health care as we know it. No hospitals, very few doctors, untreated water systems, no trash disposal, limited access to medicine (antibiotics have a very limited shelf life), etc.
That's a recipe for middle ages style pandemics regardless of a zombie virus.
So when you throw in the whole "everyone who dies will turn" concept, that's all the amped up threat we need without something like "the virus is mutating."
