Zombie

A Zombie is a hostile mob that appears in two slightly different forms. The most common form closely resembles the player, and as of snapshot 12w32a, a rarer form that resembles a villager may appear. The rarer form is called a Zombie Villager, and 10% of all zombies spawned are of this type. If a zombie attacks a villager and kills them, there is a chance (100% in Hard difficulty and 50% in Normal) that the villager will become infected and transform into a zombie villager. This often occurs during sieges.
Zombies spawn in dimly-lit areas, and will attack the player by slowly moving toward them, touching them and thereby inflicting damage. Like skeletons, zombies catch fire in sunlight.
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 Zombies will attempt to break down wooden doors, continuously banging on them. This is only successful on Hard and Hardcore though; on other difficulties, the door will crack, but not break. Iron Doors are always safe from zombie attack and destruction. Zombies will attack Villagers within 16 blocks of themselves, ignoring the hazard of direct sunlight. The Villager(s) will attempt to flee from pursuing zombies. Once a zombie has focused on a Villager, the zombie will ignore any other Villagers and the player until its target is killed or when the player attacks it.