When we think of zombies, we typically think of the dead crawling out of their graves and eating brains. This is not the case when it comes to "reality." With mutations and the world of disease and infection evolving, a "zombie outbreak" is very possible. New sciences and technology are constantly changing the world.
Diseases similar to the "zombie virus" have already emerged and wiped out billions of people. In the early 1900s, a huge plague killed a third of China. The Black Plague was the biggest infection of the world's history. Then there's Mad Cow Disease, Ebola, and so much more.
The traditional zombie originated centuries ago in Haiti. Haitians have a strong belief in voodoo and practice it religiously. Bokor, or sorcerers, would perform rituals to create zombie slaves. They would blow magic powder in the subject's face, essentially killing them. They would then bury the deceased and dig the body back up weeks later to find the subject alive and zombified. After researchers traveled to Haiti and attended these rituals, they discovered the fascinating truth. This "magic" Bokor powder consisted of seven different chemicals, two of which were the vital ingredients to create these "zombies." The first chemical is called Tetrodoxtoxin. It is the paralyzing agent in puffer fish. When ingested, the subject's heart rate slows to almost a stop, making them appear to be dead. The second chemical is from a concocted mixture of several toxic plants. It affects the functions of the brain, disorienting the person and wiping their memory. Because of the shock and confusion, the subject feels compelled to follow the directions and demands of the people around them. They don't necessarily still practice these rituals anymore, but voodoo is still a strongly-practiced belief in many different countries.
Believe it or not, insects have been living with their own zombie virus. There is a fungus called Ophiocordyceps that latches itself onto the brains of the insects that it infects. After a while, it begins to disturb their sense of self-control. Eventually, this fungus takes complete control over the insect's mind. It will compel them to make their way up a tree and latch onto the underside of a leaf so the fungus can break through the skull and grow downward to infect other insects with its spores. Now imagine a small mutation or change in its properties, allowing itself to infect bigger animals and even humans.
What if I told you that you are currently living with a parasite that causes zombie-like behavior in small animals? Because there a great possibility that you are. This parasite is called Toxoplasma Ghondii. It starts in rats, but only some are affected by it. When active, this parasite causes the rate to lose all sense of protection. Its fight-or-flight response is almost non-existent. Usually if rats smell evidence of a threat nearby (like a cat for example), it will run the other direction. Rats with this parasite won't recognize the potential danger ahead and continue on their way. These rats eventually get eaten by another animal, passing the parasite onto them. Cats, dogs, horses, and many other animals will acquire these parasites, but are not affected by them. Then we get the parasite from our infected pets. Two-thirds of the world's population has this Toxoplasma Ghondii living within them. Two-thirds! It does not affect us in any way, shape, or form... yet. But think about it; the same parasite that takes over rat's minds is probably inside of you right now. The possibilities are endless.
There's still so much we don't know about the world we live in. With everyone becoming more obsessed with the idea of a zombie apocalypse, I wouldn't be surprised if the government was secretly creating a zombie virus.