This is something I've been thinking about. What if "ground zero" was the source of some time of gas leak. Gas leaks out in mass amounts getting in the air stream. Select group of people are initially affected by the gas causing the to die and become zombified. Then the rest of the population would be eaten or infected quickly.

The gas would have to come from some type of government experiment. Maybe the virus or strain of whatever they were working reached a critical point and adapted to such a degree that it could no longer contained. Once it was out, it multiplied rapidly as it hit the airways. Like I said before, maybe the virus only initially affects a small group of people.

For this to work, there would have to be labs all over working with the same virus. This would mean that there are several "ground zero" locations. As I write this it feels weak. Sounded way better in my head.