"The Rising." That's how it came to be known eventually, the day the dead began to rise from their graves and walk once more.
It began innocently enough. Just a handful of cases located randomly across the country, of the recently deceased coming back to life. At first it was blamed on human error, the doctors must have made a mistake, what other explanation could there be? But it wasn't a mistake and it soon became clear that despite the fact that they were walking around, the recently deceased were still just that... deceased.
Essentially harmless if somewhat disturbing to see, but with no real sentience and no other signs of life to speak of the walking dead (now known as "The Risen") were placed in quarantine and subjected to procedure after procedure from teams of doctors and scientists determined to find an explanation for the phenomena. No explanation was ever found and the number of walking dead slowly increased.
Less than one month after the first reported case, the disease mutated.
A careless lab technician approaching the end of his shift and thinking more about his plans for the evening than of safety was accidentally bitten while taking a saliva sample from one of the risen. Afraid of being quarantined himself he hid his injury from his superiors and went home. He should have known something was wrong when he started feeling dizzy but he told himself he was just overworked and that he just needed to relax. He thought he'd be fine by morning. He was wrong.
By morning he was dead. Walking dead but different from the others. Self aware, though only minimally, he was governed by a hunger that could only be sated by the taste of human flesh. He attacked everyone he saw, his girlfriend, his neighbours, the mail man, the bus driver, taking a bite from each of them before being fought off. He was apprehended of course and eventually subdued but not before managing to inflict bites on several of the arresting officers.
Within 24 hours everyone who had been bitten had also become walking dead, consumed with the same hunger for flesh, and in little more than a week, the whole town had been infected. With no one left to even try and contain the contagion, it spread quickly to the next town and then the next, spreading right across the country and then the globe like a wild fire and in just a few weeks the number of walking dead was equal to the number of living... and rising.
That was a year ago. All that remains of humanity now are the small and remote pockets of survivors that live cut off from the rest of the world in secured compounds, ever watchful of the walking dead that surround them on all sides and hoping desperately that the nightmare will eventually end.