Minecraft wiki entry for Villager, associated text: When a zombie kills a villager, it can turn the villager into a zombie villager, depending on the difficulty: 0% chance on easy, 50% chance on normal and 100% chance on hard. Zombie villagers also spawn naturally in the Overworld in the same conditions as a normal zombie, although much less commonly, with a 5% chance. Zombie villagers also spawn in abandoned villages (zombie villages) and igloos, in place of villagers in zombie villages.