Weighing in the the zombie/non-zombie nature of the virus:

What if it was deadly to human beings, needing to infect them to complete it's life cycle, but made aggressive carriers out of wildlife? Birds/vermin/stray dogs etc feed on the bodies of the infected dead and the cycle renews itself. This solves the niggling problem that any police force would absolutely demolish a zombie horde, but scared, gun wielding citizens desperate for food and shelter exasperate a plague scenario.