Zombie Island was a small island located within a distance from the Keep Away Island. It mostly consists of a small featureless island with a small sandy and rocky beach which serves as the home of the simple-minded zombies.
When Peri was infected by a disease which gave him symptons making him resemble a zombie, the vigilant Two-legs Joe and Patricia hurl him via catapult to that island. When he believed himself that he felt that he was getting a little bit better, peri used the island inhabitants as a raft to get him home (as Peri explained, zombies can float). However, that meant that Peri also stupidly brought them back on mutant's island with him. (Bite, Shuffle And Moan)