When there is no more room in hell, the dead will walk the earth.
~ Someone's reason for the zombie apocalypse.

Zombies are often portrayed as mindless and cannibalistic, but they are sometimes intelligent. They are type of undead Monsters or, alternatively, a being that is not dead but unnaturally lived thanks either infection with some strange virus or a parasite that controls their mind and motor system, or simply corpses that are revived via necromancy or other dark magic with similar effects.

In other cases, zombies are also created via mind-washing at hands of Brainwashers or magical substance/influences that brought corpses (mostly).

However, not all zombies are undead. In fiction, some cases are shown that zombies can also be created via certain methods of brainwashing on innocent people that made them become mindless servants, either magically or by mind control devices; perfect examples of this are voodoo zombies, zombies that are actually humans who are brainwashed via a voodoo ritual. What's shocking is that they are also known as Homo Necrosis Zombifis, which indicates that they belong to humanity. Zombie Lords can also exist, also known as the Homo Necrosis Zombifis Dominus, Alpha Zombies, or Zombie Progenitors.