In humanities darkest hour it's greatest heroes shall arise. Such has always been true of humanity. The more opposition we face the greater we become. Our own minds work like evolution, except sped up, allowing us to learn and adapt to our ever changing surroundings. Every time we're faced with a problem they find a way not only around it, but to change so that it will happen more or less. No less was true when the world ended... or nearly did. In order to survive a few lucky among us evolved, and now they lead us to the future, where ever that may be.
It started with one virus, that morphed to create the first zombies. Not dumb zombies or slow shambling things. Ones smart enough to only eat animals, not other humans, they only bite humans, so their numbers expand faster. Ones who know how to use a boat and cross an ocean. Zombies faster and stronger than many mere humans. Seemingly the ultimate enemy, but it was only the beginning...
Okay, now that I've got your attention, let me abuse it for a second by explainin' some of the more boring bits. First off I am looking for a group, 4 or 5, maybe even more if enough people want to join and have interesting characters. The idea here is to think of X-men-like proportions for your characters.
If you don't know your X-men, here's the wiki. http://x-men.wikia.com/wiki/X-Men_Wiki
When I get my character finished and post him you can use him as an example. Think higher-tier X-men as well, someone able to take out entire groups of zombies. So while kitty is cool with her phase shifting and she can take out a few people with a bit of trickery, she'd never be able to take on a group of 5 or 6 zombies. She might be able to avoid them and run off but not take them down. It's a perfectly fine and acceptable power but just get something to go with it, I'd suggest some sort of attack, get creative!
I do have to say that Wolverine is out, he's nearly invincible and his built in claws make for basically no weaknesses. He could take out thousands, if not all the zombies in the world, by himself. The Phoenix is also out, as is the Scarlet Which and other characters like that with seemingly no weaknesses. I'm not entirely against regeneration or telepathy or what have you, just don't make it where a hundred of these things don't even phase you.
If I haven't been picky enough yet I'm about to be. I'd prefer the powers had something to do with what happened to you when the bombs dropped. It's only for lore purposes really, if the thought is that when a nuke hit your city you spontaneously evolved then it only makes since you'd evolve in a way that made since with what you were doing. If a wall starts falling on you then you might learn some super strength to hold it up. Or if your in a zoo you might learn to morph into an animal to avoid getting eaten. I will be a bit forgiving here, if you can't think of why you'd randomly gain such and such power than that's okay.
The characters don't necessarily need to start in the same city but the plan will be to meet up eventually so let's try to stick to one continent, North America. If you have some power that will let you easily get to North America from someplace else then you can start someplace else, but just plan on going their eventually.
I think that's it, if you have any questions, feel free to ask. Now, back to the lore.
Although no one knows exactly where it started many think the virus started somewhere in Switzerland. It quickly spread through Europe, with no country able to raise a solid defense quick enough to stop the disease from spreading. The Middle East fell soon after, their inability to band together causing their downfall. At that point the United States, Canada, Mexico, and a few central American countries came together to jointly close off their borders entirely. Any planes attempting to land within any of the countries were shot on site. As were ships and anyone on foot as well. China, India, and many other Asian countries, including Taiwan, Tibet, North and South Korea, and Mongolia, also came together to form a solid line of troops along the borders of the newly formed alliance against the zombies.
The zombies next moved to Africa where multiple holy wars had broken out because the apocalypse has come they said. They were devoured by the horde slowly, but surely, before the hordes massed to attack Russia. Russia's massive border was nearly impossible to defend, and they knew it, as such they stationed troops around every major population center and left the rest of the frozen tundra to the zombies. They held out a few days, but the zombies were many and the Russians were not. Eventually they lost to the tide but, as their last act of desperate hope, they nuked their own cities, denying the zombies food and more members for their army.
The new Asian Empire was next, and they were prepared, every entry by land was blocked, and they held out for just over a month before the zombies grew smarter and attacked in mass by sea. Thousands of ships flooded the Asian lines and it was the end of the war for them. With Asia gone they had billions among their ranks and it was then the U.S.A. started nuking the zombies. It was a drastic measure but no one knew any other way to survive. The zombies quickly covered the ocean to find that even in their amazing numbers they couldn't get past the United State's navy. They did make it to South America though, and they soon fell. The U.S. stationed the majority of it's land forces directly on the panama canal border next in hopes that they could stop the there. And they did. But the northern border was harder to defend, and although it fell slower than the rest of the world, the U.S. did fall.
Following Russia's example, the U.S. nuked their own population centers. No one really knows for sure why, maybe just to spite the zombies, or maybe because they have a secret reserve of troops still holding out someplace.