Zombies, along with Zombie Pigmen, Skeletons and the Wither, are considered "undead". Thus, they can be damaged by Splash Potions of Healing, are healed by Splash Potions of Harming, and are unaffected by poison or regeneration. They likewise take extra damage from any weapon enchanted with Smite. Finally, zombies, like other undead (except the wither), catch fire in sunlight. While this will eventually kill them, it also makes them more dangerous: a burning zombie can set the player (or other victim) aflame with an attack.
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http://minecraft.gamepedia.com/Zombie