A long term World War Z or The Walking Dead scenario just can't happen without the sci-fi "undead" component. A "rage virus" producing effects like in World War Z would probably devastate society, but within a week or so most of the infected would be so weak from hunger that they were not a significant threat, and a few weeks after that they'd all be dead from some combination of starvation/dehydration/exposure.
Yeah, that's the bit where I always have to remind myself that it's meant to be "hand waved" away in those stories. I watch The Walking Dead and start rambling about how they started off talking about some virus/agent that kills the brain and "restarts" just the drive to eat and kill. Even so, without the fantasy aspects, there's just no way that a living body (regardless of how crazed or hijacked by a pathogen) would still be able to move around after the organs are damaged or deprived of blood.
The closest thing to a zombie outbreak would be some sort of plague like rabies or encephalitis that makes you lose your mind and then kills you. If it was virulent enough and hard enough to treat you'd end up with huge disruptions but more like any other massive pandemic. The whole bit about victims somehow sticking around for months or years, waiting to infect more people is just impossible as far as I can tell.
