Zombies created by voodoo tend to be harmless, and are often used as slaves by the witch doctors that have created them.
In spite of its rather feeble intelligence, the modern zombie is a both intellectually and physically driven only by his all-consuming hunger for fresh human flesh. Why the dead are so hungry for living flesh is still unclear.
In the movie Day of the Dead, zombies do not need to eat, they reach for live flesh even when they have no mouth or gullet, even when their stomachs have been removed. The impulse is part of their very fibre, a spiritual craving. They are dead, and death wants to consume life. It is an image of insatiable nihilism that is hard to resist.
In living beings, the sensation of hunger is a reaction to several stimuli: the sight or smell of food, the lack of certain nutrients, and an empty stomach, to name a few. This sends a signal to the hypothalamus to produce ghrelin, considered by many scientists to be the “hunger hormone”. With ghrelin acting upon the brain, living creatures feel the need to eat, which in turn helps replenish any lost bodily cells and fuels the metabolism.
It is highly possible that the hypothalamus also experiences limited function in zombies. While it is capable of producing ghrelin to induce hunger, a zombie’s hypothalamus might not be able to produce leptin, the hormone that cancels out the effects of ghrelin. Without leptin to counteract the ghrelin, zombies are constantly hungry, the urge of tear us apart and eat us growing stronger and stronger as time passes. They may also get some relief from the hunger pains as live flesh may contain enough leptin to subside their hunger pains, cravings and need to feed. Since their body is in a constant decay stage, they become drug addicts, craving their next of leptin.