We found out recently that if you try to leave a little kid in a graveyard late at night, he'll freak out. Even if you offer to leave him a gun to protect himself. Why? It's because on some instinctual level, all humans know it's just a matter of time until the zombies show up.

Our culture is full of tales of the undead walking the Earth, from our religions to our comic books. But, some sort of zombie apocalypse isn't actually possible, right?

Right?

Guys?

Actually, yes. It's quite possible. Here's five ways it could happen, according to science.

As seen in ...
Resident Evil IV

What are they?
Parasites that turn victims into mindless, zombie-like slaves are fairly common in nature. There's one called toxoplasmosa gondii that seems to devote its entire existence to being terrifying.

This bug infects rats, but can only breed inside the intestines of a cat. The parasite knows it needs to get the rat inside the cat (yes, we realize this sounds like the beginning of the most fucked-up Dr. Seuss poem ever) so the parasite takes over the rat's freaking brain, and intentionally makes it scurry toward where the cats hang out. The rat is being programmed to get itself eaten, and it doesn't even know.

Of course, those are just rats, right?

How it can result in zombies:
Hey, did we mention that half the human population on Earth is infected with toxoplasmosa, and don't know it? Hey, maybe you're one of them. Flip a coin.

Oh, also, they've done studies and shown that the infected see a change in their personality and have a higher chance of going batshit insane.

Chances this could cause a zombie apocalypse:
Humans and rats aren't all that different; thats why they use them to test our drugs. All it takes is a more evolved version of toxoplasmosa, one that could to do us what it does to the rats. So, imagine if half the world suddenly had no instinct for self-preservation or rational thought. Even less than they do now, we mean.

If you're comforting yourself with the thought that it may take forever for such a parasite to evolve, you're forgetting about all the biological weapons programs around the world, intentionally weaponizing such bugs. You've got to wonder if the lab workers don't carry out their work under the unwitting command of the toxoplasmosa gondii already in their brains. If you don't want to sleep at night, that is.

You may be protesting that technically these people have never been dead and thus don't fit the dictionary definition of "zombies," but we can assure you that the distinction won't matter a whole lot once these groaning hordes are clawing their way through your windows.

As seen in ...
The movie The Serpent and the Rainbow, the upcoming Resident Evil 5 video game.

What are they?
There are certain kinds of poisons that slow your bodily functions to the point that you'll be considered dead, even to a doctor (okay, maybe not to a good doctor). The poison from fugu (Japanese blowfish) can do this.

The victims can then be brought back under the effects of a drug like datura stramonium (or other chemicals called alkaloids) that leave them in a trance-like state with no memory, but still able to perform simple tasks like eating, sleeping, moaning and shambling around with their arms outstretched.

How it can result in zombies:
"Can?" How about "does."

This stuff has happened in Haiti; that's where the word "zombie" comes from. There are books about it, the most famous ones by Dr. Wade Davis (Passage of Darkness and The Serpent and the Rainbow). Yes, the movie The Serpent and the Rainbow was based on this guy's actual science stuff. How much of it was fact? Well, there was that one scene where they strapped the guy naked to a chair and drove a huge spike through his balls. We're hoping that part wasn't true.

What is definitely true is the story of Clairvius Narcisse. He was a Haitian guy who was declared dead by two doctors and buried in 1962. They found him wandering around the village 18 years later. It turned out the local voodoo priests had been using naturally occurring chemicals to basically zombify people and putting them to work on the sugar plantations (no, really).

So, the next time you're pouring a little packet of sugar into your coffee, remember that it may have been handled by a zombie at some point.

Chances this could cause a zombie apocalypse:
On the one hand, it's already fucking happened! So that earns it some street cred right off the bat. But, even if some evil genius intentionally distributed alkaloid toxins to a population to turn them into a shambling, mindless horde, there is no way to make these zombies aggressive or cannabalistic.

Yet.

As seen in ...
28 Days Later

What is it?
In the movie, it was a virus that turned human beings into mindless killing machines. In real life, we have a series of brain disorders that do the same thing. They were never contagious, of course. Then, Mad Cow Disease came along. It attacks the cow's spinal cord and brain, turning it into a stumbling, mindless attack cow.

And, when humans eat the meat ...

How it can result in zombies:
When Mad Cow gets in humans, they call it Creutzfeldt-Jakob disease. Check out the symptoms:

• Changes in gait (walking)
• Hallucinations
• Lack of coordination (for example, stumbling and falling)
• Muscle twitching
• Myoclonic jerks or seizures
• Rapidly developing delirium or dementia

Sure, the disease is rare (though maybe not as rare as we think) and the afflicted aren't known to chase after people in murderous mobs. Yet.

But, it proves widespread brain infections of the Rage variety are just a matter of waiting for the right disease to come along.

Chances this could cause a zombie apocalypse:
If the whole sudden, mindless violence idea seems far-fetched, remember that you are just one brain chemical (serotonin) away from turning into a mindless killing machine (they've tested it by putting rats in Deathmatch-style cages and watching them turn on each other). All it would take is a disease that destroys the brain's ability to absorb that one chemical and suddenly it's a real-world 28 Days Later.

So, imagine such an evolved disease, which we'll call Super Mad Cow (or, Madder Cow) getting a foothold through the food supply. Say this disease spreads through blood-on-blood contact, or saliva-on-blood contact. Now you have a Rage-type virus that can be transmitted with a bite.

Just like the movie. With one bite, you're suddenly the worst kind of zombie:

A fast zombie.

As seen in ...
Laboratories around the world.

What is it?
You know all that conversy out there about stem cell research? Well, the whole thing with stem cells is that they can basically be used to re-generate dead cells. Particularly of interest to zombologists like ourselves is neurogenesis, the method by which they can re-grow dead brain tissue.

You can see where this is going.

How it can result in zombies:
You wanted the undead to make an appearance in this article? Well, here you go, you creepy bastards.

Science can pretty much save you from anything but brain death; they can swap out organs but when the brain turns to mush, you're gone. Right?

Well, not for long. They're already able to re-grow the brains of comatose head trauma patients until they wake up and walk around again.

Couple that with the new ability to keep a dead body in a state of suspended animation so that it can be brought back to life later, and soon we'll be able to bring back the dead, as long as we get to them quickly enough.

That sounds great, right? Well, this lab dedicated to "reanimation research" (yes, that's what they call it) explains how the process of "reanimating" a person creates a problem. It causes the brain to die off from the outside in. The outside being the cortex, the nice part of you that makes humans human. That just leaves the part that controls basic motor function and primitive instincts behind.

Reanimation research (artist's rendering)

You don't need the cortex to survive; all you need is the stem and you'll still be able to mindlessly walk and eat and enjoy Grey's Anatomy. This is how chickens can keep walking around after they've been beheaded (including one case where the chicken lived for 18 months without a head).

So, you take a brain dead patient, use these techniques to re-grow the brain stem, and you now have a mindless body shambling around, no thoughts and no personality, nothing but a cloud of base instincts and impulses.

That, ladies and gentlemen, is what we like to call a real, live, undead fucking zombie. So there.

Chances this could cause a zombie apocalypse:
Think about it. Under every legal system in the world, all rights and responsibilities are terminated at death. All it takes is someone with resources and a need for a mindless workforce of totally obedient slave labor.

How long until somebody tries this? We're betting somebody in the world, maybe North Korea, will have a working zombie by Christmas.

As seen in...
Michael Crichton's novel Prey, The PS2 game Nano Breaker

What are they?
Nanobots are a technology that science apparently engineered to make you terrified of the future. We're talking about microscopic, self-replicating robots that can invisbily build--or destroy--anything. Vast sums of money are being poured into nanotechnology. Sure, at some level scientists know nanobots will destroy mankind. They just can't resist seeing how it happens.

How it can result in zombies:
Scientists have already created a nano-cyborg, by fusing a tiny silicone chip to a virus. The first thing they found out is these cyborgs can still operate for up to a month after the death of the host. Notice how nano scientists went right for zombification, even at this early stage. They know where the horror is.

According to studies, within a decade they'll have nanobots that can crawl inside your brain and set up neural connections to replace damaged ones. That's right; the nanobots will be able to rewire your thoughts. What could possibly go wrong?

Chances this could cause a zombie apocalypse:
Do the math, people.

Some day there will be nanobots in your brain. Those nanobots will be programmed to keep functioning after you die. They can form their own neural pathways, meaning they can use your brain to keep operating your limbs after you've deceased and, presumably, right up until you rot to pieces in mid-stride.

The nanobots will be programmed to self-replicate, and the death of the host will mean the end of the nanobots. To preserve themselves, they'd need to transfer to a new host. Therefore, the last act of the nanobot zombie would be to bite a hole in a healthy victim, letting the nanobots steam in and set up camp in the new host. Once in, they can shut down the part of the brain that resists (the cortex) and leave the brain stem intact. They will have added a new member to the unholy army of the undead.

Now, it should be more than clear by this point that our goal is to be responsible researchers. We don't want to create a panic here. All we're saying is that on an actual day on the actual calendar in the future, runaway microscopic nanobots will end civilization by flooding the planet with the cannabalistic undead.

Science has proven it.