This chart explores the possibility of a zombie virus attack the USA.
Let us begin with the assumption that the virus hits the cities with the largest immigrant population. Winter keeps the virus contained in the northern regions.
As the independent civilian population begin to the west, they carry the virus along. This goes on for 90 days and after which there is the onset of spring. This causes the virus to spread into every major city of the country.
This process takes about half of the year to complete. From here on the spread is so rapid that the entire country is left to fend for itself. Now the government withdraws itself from the affected states and by the end of the year the zombies begin to attack crops.