Are Zombies Real? 10 Reasons the Undead Can't Walk Among Us

Zombies are undead figures who feast on human brains and flesh; they are a fixture of folklore and popular culture. As such, writers and scientists have theorized on the different ways that a zombie apocalypse could come to pass. Sometimes it's a virus. Other times it's a result of radiation exposure. And then there's the parasitic fungus.

Zombies from 1968's "The Night of the Living Dead" were actually called ghouls, but they definitely exhibited many of the virtues of what we consider zombies. They slowly but relentlessly clawed their way toward any breathing person they could find, making up for their lack of speed with ceaseless patience and overwhelming numbers.

More modern zombies, such as those from 2013's "World War Z" might be a reflection of our faster-paced, internet-fueled societies. They're undead, yet they're also capable of running down slow-footed victims, and they exhibit flickers of intelligence, too.

Zombies are a physical paradox. They're undead, yet they move around like they're alive. (Some are slow-moving, but there are also fast zombies.) They're cold and lifeless, but somehow they crack open skulls to dig out a dessert of brains. They're rotting away but also stumbling down city streets grabbing unlucky people left and right.

In the next sections, we'll explore 10 reasons zombies could not exist in the natural world.

Ever been to Atlanta in August? The word "stifling" doesn't quite capture the misery of triple-digit temperatures paired with humidity levels higher than a hothouse. On the flip side, North Dakota in January is on the hellish side, too, freezing living tissue in minutes and killing just about anything crazy enough to wander outdoors unprotected.

The long and short of it is that Earth's unrelenting weather would take a toll on human zombies in several ways.

High heat and humidity speed the deterioration of rotting flesh by providing perfect conditions for the proliferation of insects and bacteria, which decompose anything they set their enzymes to. The dry heat of a desert would suck the reanimated corpses dry as husks in a matter of hours.

The bone-cracking depths of winter would cause zombie bones to become more brittle and fragile than they already are. Even the slightest blow or stumble could make their skeletal systems completely collapse, perhaps even under their own weight.

That's not to mention the deterioration that ultraviolet sunrays, hurricane-force winds, sheets of rain and hail, or mountains of snow could cause. Of course, all of this foul weather may be why so many zombies prefer the relative safety of basements, dungeons and abandoned prisons.

We're all mechanical animals of sorts, our locomotion made possible by the links between muscles, tendons, skeletal elements and much more. When part of that system goes awry, we don't move much, if at all.

That makes it all the more puzzling that so many modern-day zombies are perfectly capable of movement even when their flesh and bones are dangling and flopping.

And yet, there is so much confounding zombie behavior. They stagger around — sometimes with frightening speed — seemingly oblivious to the impossible physics driving forward their rotting, shredded muscles and broken bones. That's even before you consider their lack of brains.

The human central nervous system controls all of our muscular activity by firing electrical signals from the brain to muscle cells, which twitch in response to the gray matter's commands.

Many zombies appear to suffer from massive head wounds that would result in brain damage, making the idea of forward motion all the more implausible.

Viruses, fungi, bacteria and other microscopic invaders have plagued humankind since the beginning of time, shortening our lifespans and often making our lives miserable. Yet it wasn't until the 1800s that we finally figured out that the tiniest of invaders, such as smallpox or HIV, are often the most dangerous of our biological enemies.

Our immune systems, brimming with the weaponry of white blood cells, lay waste to infections and keep us alive ... for a least a while. People who suffer from immune system deficiencies struggle with all sorts of problems.

That is the plight of zombies, too, because they have no immune systems to speak of. With their ample fleshiness, zombies are a perfect breeding ground for untold numbers of bacteria, fungi and viruses that would make short work of their hosts, devouring them from the inside out.

And seeing as how they're zombies, that's probably a good thing.

We humans eat food so that we can convert chemical energy into the activities that keep us alive, from breathing to reproduction. It's our metabolism that maintains these processes. Metabolism is an overarching term encompassing all of the chemical reactions that happen within our bodies.

In theory, zombies consume brains because they, too, require sustenance that perpetuates their ability to function. There's just one problem — zombies aren't actually alive. As members of the undead society, they lack metabolic capabilities of any kind.

The nutrients that humans consume begin to break down the moment we start chewing up a slice of pizza or quiche. Our stomachs take over from there, converting those nutrients into calories that we need to stay alive.

Zombies, on the other hand, have no metabolism at all.

Even if they could magically summon the energy to chomp on some tasty brains, their stomachs (if they even have stomachs) couldn't provide a chemical pathway for nutrients to convert to energy, leaving the undead rather inanimate and, well, lifeless.

Hyenas, wolves, bears, coyotes, foxes and packs of vicious feral dogs — when the apocalypse strikes, you'll have just as much to fear from these predators as you do zombies. They're fast, sometimes ferocious, and when they're ravenous they become bolder and much more willing to attack healthy humans.

So how would these animals react to the sight and smell of the reanimated dead, which are essentially walking bags of meat?

In an ecosystem ripped apart by the chaos of a zombie invasion, hunger is sure to be a challenge not just for the remaining humans but also for their animal counterparts. And like humans, those animals will do just about anything to survive, even if it means nibbling at the festering, crawling remains of what's left of someone's loved one.

It's not just top-tier predators that would attack, either. Smaller animals like rats, raccoons and possums would also happily munch on undead goodies if given the opportunity — such as a legless, armless zombie sprawled in a mud pit, awaiting its final, sorrowful end.

Vision, hearing, touch, taste and smell are all key to our survival. Without our five senses, we'd roam this Earth aimlessly and for a very short time, eating poisonous plants, slamming our heads into every door frame and undoubtedly stubbing our toes on every coffee table, too.

Because zombies are perpetually disintegrating, it's difficult to understand how they'd perform any of the vital actions necessary to hunt yummy brains.

As they began to rot, the soft tissue of their eyes would be among the first organs to fall apart, leaving zombies grasping blindly for anyone unlucky enough to wander into their paths.

Their eardrums would warp and tear and fall in tatters, as would the rest of their auditory system. Deaf and blind, zombies would then fall back on their sense of smell, which their own stench would likely overwhelm.

That means zombies would need to feel their way through the world. In large numbers, a few would certainly nab victims from time to time, but fully functional humans could avoid these monsters in most situations.

Nature has devised some ingenious and horrifying ways to spread germs. Take measles, for example, which spreads by coughing and sneezing.

It's a disease so contagious that around 90 percent of people who come into contact with an infected person will also become ill [source: CDC]. It's an insidious virus, too, as it can live for around two hours outside the body, hanging in the air, just waiting for a hapless person to inhale it and start the replication process anew.

Then there are zombies, which have to bite people to spread their contamination. There are an array of issues with this sort of propagation, starting with the fact that it's terribly inefficient.

First, the zombie has to somehow grab a person long enough to inflict a bite. This is a tough proposition for a creature that may be missing an arm or leg, making it challenging to chase down terrified prey. Second, the act of biting consumes enormous amounts of time and energy, two things that rotting zombies do not have.

And three, biting requires close physical contact with a victim. In a time when survivors will be ever-vigilant and hard to find, those cold and slow zombies will be hard-pressed to press flesh with any of their warm-blooded cousins.

Before the advent of antibiotic creams and pills, simple scrapes and cuts were a treacherous endeavor even for healthy-as-a-horse humans. Cuts allow dirt and germs to get a little wiggle room and make their way into your innards.

But with proper hygiene, including wound rinsing and hand washing, most people eventually recovered from these kinds of injuries, although the healing process took longer and sometimes resulted in long-term side effects.

Whereas humans have tissues that can regenerate and heal themselves, zombies do not. Their wounds, no matter how mild or severe, are permanent. Imagine, if you will, a paper cut that not only doesn't heal but actually becomes wider and deeper by the day. As the flesh continues to divide, more bone is visible, and eventually, the last drooping flaps of flesh fall to the ground.

A small scrape would blister into peels of skin that sag and slide right off of a zombie's body. Skin charred by burns would curdle into a goopy, grimy mess. And all of this decomposition would be for the best — no one wants to be a zombie for long, anyway.

Your stomach is a muscular bag that has a capacity of roughly 30 ounces (0.89 liters) of liquid and solid matter. People who eat large meals regularly may stretch that capacity to some degree. And those who go on steep diets may actually shrink their stomachs.

Suffice it to say that zombies are not likely to join Weight Watchers anytime soon. Zombies are the competitive eaters of the monster world, inclined to stuff themselves at each and every brain buffet they can find.

There are some problems with this style of consumption, and it has nothing to do with a zombie watching his figure. Instead, it has a lot to do with wherever that food winds up.

Because zombies aren't known for their fully functioning bodies, they may very well have gaps in their digestive system somewhere between their mouths and anuses. Gaping holes in this route make it impossible for zombies to derive any sort of benefit from their meals at all.

Of course, because so few parts of zombies' bodies work, there's a good chance that the brains they eat will just hit a dead end (pun intended) where the esophagus hits the stomach.

As the zombie dines on more and more brains, the undigested glop of chewy stuff would just ferment, bubbling gases and eventually bursting through the walls of the zombie's stomach.

The enamel on your teeth is the hardest substance in your body. This tough shell protects your chompers from the rigors of chewing foods, and with proper dental care will last you for a lifetime.

The key phrase there, of course, is that one about proper care. Guzzling sugary soda and neglecting to floss sets you up for decades of misery, including cavities, receding gums and a mouthful of other awfulness.

Zombies typically don't brush their teeth twice per day, even if they have any teeth at all. As their gums rot and the enamel cracks and fades, the bony protrusions of their teeth will become pitted and stained and then fall right out of their skulls, making biting you a hopelessly futile endeavor.

Still, a zombie's teeth are probably about the last part of its body to go. Even if those teeth shatter and break, they'd still make for formidable weapons if you were careless enough to stumble into a zombie's death grip.

But of course, that probably won't happen. For the time being, no virus or radiation leak or fungal infection has caused the world to be overrun by and undead horde.

On the bright side, you'll probably never have to run for your life as hundreds of dead people stumble and mumble and groan all around you and your family, clawing at your face and trying to eat your brains. Sadly, it also means you'll never have the chance to put your sophisticated zombie survival plan into action.

Either way, zombies are a physical impossibility. Nothing short of some pretty amazing magic would make them a real threat to humanity.