Many moons ago, there were four Clans: BirchClan, LightClan, LeafClan, and RushClan. They had lived in peace in their territories for generations, without any trouble. That is, until the disease struck. It started out as one sick apprentice, who ate a bad rabbit. Then it spread and spread, until not even the most experienced medicine cat could heal it. All four Clans were wiped out, and the survivors fled and became rogues or kittypets. The Clans were forgotten.
Many moons later, closer to our current time, StarClan decided it was time for the Clans to return to their territory. They sent dreams to the ancestors of the former Clan cats, explaining what happened and giving cats their Clan names. Some of them are starting to return to the Clans, and rebuild the four Clans from scratch.
Right now all the cats are meeting at Fourstones, where they will stay until they have enough cats to remake the Clans. Leaders and medicine cats will be chosen by StarClan, and the cats will split themselves up and return to the territories.