When Rick Grimes opened his eyes after a medically induced coma, the world he knew had already disappeared. The sheriff's deputy had been shot during a routine police operation with his best friend Shane Walsh, but while Rick was unconscious, civilization collapsed. The cause was the mysterious Wildfire virus, first identified by the CDC on April 16, 2010. The airborne infection left people as silent carriers, but after death, it reactivated the brain and transformed the deceased into walkers.