Zombies on the Rise
This link promotes the Zombie Survival Guide book. Do check out the links, which have zombie infection simulations for cities... like this one here.

I recommend the nuclear death variant, which allows you to nuke city blocks to kill off infected areas. It's tough... the damn things replicate so fast! But a good strategy is to contain the buggers by bombing the area around the zombies, then killing them off inside.

I rather like the zombie movies as an inspiration to an RPG session. Zombies normally provide nice dangers for combat-hungry players... the breakdown of basic societal and organizational norms normally becomes a great opportunity for role-playing horror... and trying to figure out how to stop the zombie onslaught and what caused 'em normally challenges the puzzle solvers of the group.