Minecraft wiki entry for Village_mechanics, associated text: In addition, if a zombie villager is cured, it gets a permanent discount on its trades. Therefore, if on Hard mode, it may be desirable to intentionally infect one's village with zombies and cure the villagers to get very good discounts. However, this takes a lot of potions and golden apples, and the zombie villagers must be kept out of the sun, so this is only recommended for late-game players.