There had been an outbreak amidst humans, a plague, infection, whatever you can use to define it. Pokemon were all immune to this infection, the humans turned into zombies of course in this world there has never been a suggest of it ever happening or the mere existence of it in fiction. So they just called the undead things. You we're in New bark town when it started, you have no idea what had happened or why you don't remember, your family is no where to be found and most houses are broken down.