You've got a fictional zombie apocalypse on your hands.
There are many ways it could begin.
In the movie 28 Days Later, it's simply a virus. In the Evil Dead movies, it's black magic from the pages of an evil tome. In video game Resident Evil, it's brain parasites. In Stephen King's novel Cell, it's sparked off by a white-noise phone call to your mobile that turns your brain to scrambled eggs.
It could be a curse, voodoo, military experiments - but whatever the reason, eventually the human population turns into mindless shambling cannibals, and you've got a fictional zombie apocalypse on your hands.
Read the Ultimate Guide to Spotting a Zombie here