I definitely agree that the ZA is ripe for unsanitary conditions and it's very likely that all kinds of nasty stuff would be floating around. However, one of the major factors for a virus to spread worldwide is people traveling, be it by boat or air. That's one thing that wouldn't be happening in a ZA.
One thing that hasn't really been explored on the show is people dying from other diseases that could be otherwise treated with medical care.
This is something I've wanted to see for quite a while. I can't remember what show I saw it on, but it was in the vein of Life After People except with a small amount of survivors after an apocalyptic event. It followed a man and his family as they talked about the kinds of threats he would have to deal with to survive, gave advice and tips, that sort of thing. Well, they made it and after a decade or so, it showed the family had banded together with others, made a small settlement and discussed how they could utilize the same sort of shite early settlers used to thrive. Life was good. He was a leader in the community. He and his wife had another child. His son had grown up and was teaching the others about farming or something important. They had a feast and there were smiles of contentment all around. It showed a tiny, non-descript cut on man's hand as he ate, then abruptly said he died from tetanus a week later. The end.
Made it through all that shit, and died of tetanus. That's the kind of curveball I'd like to see on the show.