The American West Saved From Zombie Apocolypse by Great American Desert

I started my zombie infection in NYC (Sorry Carlos) and let it run for a considerable time (77 days simulated).  The infection rate dropped to the point where the sim stopped.  I tried adding a couple more zombies in the edge cities which ought to go next and the infection still stopped.

Its a fun little sim you can leave running in a tab in your web browser.

For the edge effects, I imagine the Mexicans and Canadians (mostly the latter) sat there with BFGs shooting any Yanqi which came within 100 ft of the border.  Alive or undead.

So, World War Z, the book version, actually has some epidemiological support for the US rallying from California.  And the American West was saved by the Great American Desert.