Zombie Science: Are Zombies possible?

I am a huge fan of everything that has to do with a Zombie apocalypse scenario. It is simply fascinating to me, although I wouldn't want to experience it myself. While reading and watching 'The Walking Dead' or playing 'The Last of Us' I have always wondered one thing. Is it even possible for Zombies to exist in the real world?

At first it all seemed quite scary, the dead rising to eat the living, and every night the week after watching it I suspected one of those rotten bastards in every dark corner or behind ervery door of my apartment. But then I started thinking about wether or not a Zombie apocalypse would really be possible in real life and after thinking about it a while, I wasn't scared anymore. This could also be due to getting used to watching Zombie movies and series and the realization that one Zombie isn't too dangerous. They are slow and dumb, you can always outrun them, just dont get surrounded.

I started thinking about that question and took a scientific approach that is by no means professional or proven, just what I came up with. I broke my thoughts about that topic down into several parts or rather several aspects of it, so this will turn into a small series of Zombie posts.

When I saw the trailer to Naughty Dog's game The Last Of Us, I learned that the developers were inspired by a parasite, or a fungus to be precise, that controls ants in a zombie like manner as part of its life cycle. In the game a similar parasite targets humans and a zombie apocalypse breaks lose. I started questioning if that would really be possible and this is what I came up with:

Parasitic mind control on a human has not been observed so far, but principally it is possible for a parasitic life form like a virus or a fungus to infect a brain in order to control its host.

Examples of this phenomenon are the eggs of the liver fluke, a worm like parasite or the fungus called cordyceps. They both target ants and force the still living insect to climb plants and grab hold of them with their jaws.

Even though the human brain is far more complex than the brain of an ant, it seems possible that typical primitive Zombie behavior or insticts can be triggered by a parasite in the brain. Also a parasite might be able to suppress memories or the conscious mind.

While it appears possible for a human Zombie to be created through a parasite, the parasite's motives remain unclear. As in the examples mentioned before, parasites typically take action with the goal of reproduction. When a victim is bitten or scratched by a Zombie, it is infected and becomes a Zombie itself shortly afterwards. At the same time the human resembles the main and seemingly only food source of the Zombie and therefore of the parasite. This means the parasite / Zombie can not survive without eating since the parasite is either feeding on the eaten flesh or the host itself or is at least dependant on a functioning host body. How long the life span of a Zombie / the parasite without food intake is can only be speculated. Given a too long life span, a too rapid infection rate and the assumption that Zombies do not feed on each other, the risk of the extinction of humanity is high, ultimately meaning extinction of the parasite aswell.

If said parasite would occurr naturally there is a good chance that nature would implement a system of regulating its population (similar to the Lotka-Volterra-Laws) to avoid its own extinction. Regulators could be for example the infection rate (hunger), the parasitic life span or under certain conditions canibalism.

Therefore I concluded that the possibility of real Zombies exists, but it is very unlikely.