One day, the world's dead revived and began attacking people.
Their victims were then resurrected into the walking dead (Walkers), only to attack humans again.

Though but a flicker at first, it soon consumed the world like the shadow cast by the setting sun. Half of the world's population would perish in the three months after the first resurrection, with 99% dead only half a year later.

Miraculously, the first revival coincided with a worldwide sterility.

An inability to understand why the dead were coming back to life meant that humanity was unable to discover a cure or take effective measures to halt infection. Some said it was the devil's work, while others claimed that it was divine retribution.

After nations and cities ceased to function, the few remaining survivors scavenged food and supplies from towns in a bid to survive. As searches yielded fewer and fewer supplies, the survivors were forced to pillage from others.

The wisest survivors established colonies to ensure mutual survival and built walls around them. In a few years time, some were even able to find solace in the restoration of basic urban functions...

But that was only the beginning of the end.