cracked.com:

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.

Brain Parasites

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.

If you woke up tomorrow and the zombie apocalypse had begun, what would you see when you checked your computer? What about the next day, and the week after that? What would the news from CNN, Al-Jazeera, and the Wall Street Journal look like? What would the most popular downloads on the Pirate Bay be? And how long before we figure out that zombie can't survive a sword to the head?
For the last thirteen days, reddit user Vidzilla has been posting image compilations that look like screengrabs from familiar websites — Twitter, Flickr, MSNBC — except they show the slowly unfolding story of a global zombie pandemic. It's remarkable that Vidzilla can get so much across without any additional explanatory text. We get the response from various governments, how initial coverups and misunderstandings eventually shift to worldwide understanding, how opportunistic con men try to sell desperate people on cures for zombieism, the black humor that accompanies such a complete disaster. How many days will the Vidzilla apocalypse go on before there's no new news online?

source cracked .com