So the first half of 'mind-blowing yet feasible' ideas is over, welcome to the world of crazy. The walker virus was planted on Earth by aliens, of course. We live on a planet rich with life and resources, regardless of what environmentalists would have you believe. Several fans have theorised that aliens want Earth's natural materials and their way of wiping out the planet was by unleashing a plague among the population. No human can work out where it came from, what is actually is, how to stop it, or how every single person on the planet was seemingly infected. Therefore the logical conclusion is... Aliens.