Well I read that cracked article, and I gotta say, while all those things look good on paper, they would never actually work. Lets go down the list and say why, shall we?
1: The parasite. This one COULD happen, but here's the thing with it; it wouldn't work. Why? Because there's no motivation and no real way to do host to host spreading. With the parasite mentioned in the article, toxoplasmosa gondii, the parasite needs to get from one organism to the other. However, there is nothing on earth that would/could eat a human that exist in the quantity required for this to work for this to work. Additionally, how would it spread from one person to the next? Even if this were to evolve into something that affects humans on a large scale, why don't the infected turn on eachother, and why would they ever form a horde with a common goal?
2: Chemical zombies. Despite the fact that this has kind of already happened, it's not a viable way to make a horde. There's no way to spread it or to control the existing zombies in any sort of useful way.
3: The rage virus. Ok, this one has the mindlessness and the spreadablilty down, but absolutely fails at the motive level. Why would any of the zombies go out and hunt the uninfected? There's just no reason to go out and find an clean person when your rage could easily be directed at the nearest zombie. so while this one could happen, there's nothing to keep the horde from ripping its self apart.
4: Stem cells! This one also fails because of motivation and spreadability. What makes the zombies attack the uninfected, and how does it spread? The answers are 'nothing' and 'it doesn't.'
5: Nanobots. The only one on the list that has motive, spreadability, and control covered. But this one fails because it would be easy to deal with, since you'd be dealing with tiny machines. Just a little bit of emp, and BOOM, problem solved. Also, where would the nanobots get the materials to make more of themselves? And if they were being controlled, it should be simple to destroy the control center and again, problem solved.
So there you have it. That is why none of that stuff would work for zomb-pocalypses. However, there is one way I could see a zomb-pocalypse working, and that is with
6: The Flood. No, not water, I'm talking halo. Those little parasite things that want to take over the galaxy. They have every aspect of the zomb-pocalypse down. They spread by infecting the living/reanimating the dead with their spores, which are made from gathering biomass. They are controlled by a single entity, so there's no reason for them to attack each other. And then they want to take over the galaxy, so there's the motive for trying to infect the uninfected. There's just one flaw; they don't really exist. But that is the way it would have to go down, or at least something like that.
So there you have it, why there will probably never be a zomb-pocalypse. Well, at least not in our lifetimes.
Concept, out