Blondes one of the Webisodes (linked above) shows the very early days of the outbreak, where the army are still buzzing around in helicopters, and making loudspeaker broadcasts.
The theory from the people in that episode was that it was a terrorist attack, some sort of germ warfare.
Which, if true, might suggest that it's confined to the USA and the enemy state might hold a vaccine....
I doubt TWD will ever venture beyond Georgia though.
Just remembered, the episode before the mid season break - didn't they hear something on the radio...? Are they about to find other survivors I wonder?