Minecraft wiki entry for Village_mechanics, associated text: 5% of zombies are zombie villagers, so it shouldn't take the player too long to find two that are curable. Additionally, when a villager is attacked by a zombie (any zombie) they have a chance (50% on normal difficulty, and 100% on hard) of turning into a zombie villager instead of just being killed. Zombie villagers are a great way to start an artificial village, because unlike villagers, zombie villagers follow the player long distances. Then when they arrive at the desired location, they can be cured.