Minecraft wiki entry for Development_versions, associated text: Villagers have a chance of turning into zombie villagers after being killed by a zombie in hard difficulty.
Zombie spawn eggs may spawn zombie villagers.
Zombie villagers can naturally spawn.
Zombie villagers can be cured, becoming normal villagers again.
To do this, first use a splash potion of weakness on it. Then, right-click on it with a regular golden apple. After several minutes, it will turn back into a normal villager.[2]