Unfortunately, as awesome as zombies are, there are many flaws to their very existence that makes any chance of a zombie apocalypse moot. The most recent pop culture reference to zombies and their pestilence upon mankind is found in AMC's series "The Walking Dead" adapted from the popular graphic novel by the same name. In this show we learn many things about the existence of zombies (but we still are not informed on their origin). We know that zombies are brought about by some worldwide epidemic that wiped out much of the human race (not bad science as we are all very familiar with the Plague, that wiped out half of the population of Asia and Europe during the 1300's). When this "infection"(note 1) invades the human host it causes high fevers, pains, and eventually massive end organ failure and death.
However, as time progresses, the "infection" then restarts the brain, but only the brainstem leaving the rest of the brain dead. For those not familiar with anatomy and physiology the brainstem consist of two components called the Pons and the Medulla Oblongata. These two sections of the brainstem are vital for normal physiological functions including respiration, heart rate, digestion, and other basic functions of life that go on within us without our conscious effort. The rest of the brain, the convoluted region, is used for higher level thinking processes such as hearing, speech critical thinking skills, motor control (motor coordination is controlled by the cerebellum), etc. Our memories, thoughts and even our very "existence" all takes place within the cerebrum. Again, the premise behind the zombies in "The Walking Dead" is that the infection kills the person and restarts the brainstem but not the cerebrum. This leads to a animate being only capable of basic functions to maintain life. However, "the you" as they call it is dead and gone, never to return. So, the zombie lives on always on the search for food.
While watching the television series we are introduced to a myriad of different zombies, all different in regards to how "whole" the zombie is. The first episode shows a zombie crawling through the park with one leg missing and the other complete devoid of flesh, just a femur hanging on. Other zombies have missing limbs, missing abdominal cavities, and others are just heads without the rest of the bodies. Yet all of these zombies still seem to be "alive".
Remember, to kill a zombie you have to have a direct blow to the brain. Cutting off an arm, a 12 gauge shell to the chest or even decapitation does not kill the zombie. The flaws found in these "living" zombies are that many of them lack vital organs and tissues necessary for basic function. The brainstem is an important factor and it is what controls basic functions, but it does not carry out those needs. The brainstem does not pump blood which carries necessary nutrients and oxygen needed for muscles to contract which then cause movement. The brainstem does not digest foods which are then trasnported by blood to muscles and organs necessary for energy production. The brainstem does not control coordinated muscle movements necessary for a zombie to grab onto a victim and pull them towards gaping jaws of eternal death. Just like the engine of the car is vital for the car to move forward, it does not transport gasoline from the fuel tank, that is the purpose of the fuel pump and fuel lines. Without those components the engine will not start. The engine does not turn the wheels of the car, that is the job of the transmission. Without a properly functioning system the engine is just as dead as the rest of the car. Without lungs to breath oxygen, a GI system to digest foods and a heart to pump blood, the brainstem is just as dead as the rest of the body. Without a proper neurovascular system the brainstem cannot receive precious nutrients nor can it send signals to the rest of the body for actions such as movement. However, there are bodyless zombie heads that still chomp trying to find sustenance for an endless hunger.
In "The Walking Dead" the survivors are forced to take refuge from these zombies, or "walkers" as they call them, and to also kill them for the groups protection. They have become extremely good at taking out individual walkers and are getting better at taking out groups of them. However, I would argue that the majority of the head shots taken by the group are not kill shots as the show would have us believe. If you pay attention to where the majority of the shots are placed, they usually are right above the eyes in the forehead area. On a normal human being this would be a kill shot, but not for the walkers. Remember, the prefrontal cortex and the rest of the cerebrum (where the shots enter and travel through) are not alive according to the "science" in the show. So, a shot to that area would be just as effective as a shot to the torso or extremities. The path of the bullet or edged weapon must go through the brainstem, which is located roughly between the oropharnyx (mouth and throat) and the nasopharynx (nose). Given the fact that the walkers' mouths are gaping open ready to ingest your highly coveted flesh and organs, this should make for a semi easy shot, but everyone still seems to go for the forehead.
Hopefully this short little anatomy lesson has calmed your fears in regards to the pending zombie apocalypse. There is no need to worry about your loved ones rising from the dead with an insatiable, cannibalistic appetite only to realize that while you are still a living, breathing human being, you are already numbered among the masses of the walking dead. While zombies may never come to fruition the real fear lies in the highly plausible, and possible super infection that quickly spreads across the earth causing a global wide pandemic.
 Thank you to Chad for finding the links to the most probable cause of "zombie-like" behavior, however, the dead to not come back alive with this disease.
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