The wither is a earth spirit that lives underground. By looking into the eyes of it's victim it can steal their soul and turn them into a zombie under their command. The zombies themselves are immune to any damage as long as their spine and/or head are intact. The zombies do feel pain. The zombies bleed heavily during their transformation and their blood are infectious and causes other people to turn into zombies. If the blood of a zombie enters a blood vessel or comes in contact with a mucous membrane, a person is infected. The wither itself is not immune to damage like the zombies. They are different creatures.
