Yeah, trying to use real medical and biological rules for zombies quickly becomes a losing proposition. WW-Z tried mitigate this a bit - the black ooze/blood that is toxic to all other life, including other microbes and parasites, and seems to keep them going even in deep ocean pressures - but the more you know, the harder it is to buy a "scientific" explanation. I think a key point is that they didn't know any of those things at the time; as somebody else said, it was early in the war, and Brooks devotes a lot of time to all the bad information floating around. Ironically, the same thing that makes zombies biologically implausible is what makes disastrous counter-strategies more likely; the authorities simply can't/won't accept that these things are reanimated corpses that can only be stopped in a very specific way. It's just "African Rabies", or whatever. Having said that, I do agree that really, you would need mass infection throughout the population, then the zombie stage later, in order for this to get really out of hand. Especially if people reanimate automatically on death, and it wasn't known at first. Hospitals might be the first places to get overrun, actually.