Idea the Second: Zombie Nation
On September 6th, 20XX, a plague was released in key areas of each country, infecting 98% of the population that had reached puberty and leaving those who had not completely uninfected. This plague, nicknamed the Gray Death, was a wasting disease. Victims of the Gray Death initally suffered only from a nagging cough and the chills. Over the course of anywhere from 3-5 days, the victim's skin took on an ashen-tone, any wounds they got wouldn't heal, and nothing would satisfy their hunger.
No adults, no one alive that was older than 12, no one able to respond to a completely unexpected threat... the deceased victims of the Gray Death rose from the grave. Those that fell victim to the vicious bite of the Grays (as they have been called) became infected themselves, dying in a few days after initial infection and rising a few days later to swell the ranks of the undead.
18 years later, the survivors of the Gray Death and the horrors that followed have developed their own mini societies... some have flourished, and some are struggling. The constant fight to ward off famine, disease, and the remaining Grays is more than some can handle. Finding the right place to begin anew requires someplace isolated, easily defended, and capable of sustaining a society one way or another.
This is the state of the country. Due to failing communications and the lack of people who knew how to fix it, no one knows the state of the rest of the world. My plan is to craft an area, probably san francisco and nearby suburbs, as well as the bay. I'd use the current status of that area as a starting point, with changes (in most cases, drastic changes) that reflect how the area would be affected by the events described above.