In the webisodes a terrorist attack was meantioned, and at the end of series 2 it is revealed that everyone is infected, and they will reanimate after death. There would have been a further explanation to the outbreak in a planned webisode. Rember in series 1 when rick jumps in the tank and there is a walker in there? Well this was to be ending of the planned webisode the walker was a US ranger in Atlanta city trying to evacuate the population during the outbreak but the city was overran. The webisode was shelved due to budgeting issues, which is a shame because it would have gave us a picture of what happened prior to rick waking in the hospital.