I've always been fascinated by zombies. Just the concept alone. Surely, it must be possible. With the astounding varieties of parasitic creatures that exist today, there must be one that can replicate the zombie image. So, rather then actually research the topic (Which I find tedious and not as much fun as designing my own), I'm going to design my own. A parasite that would have to completely enslave the human mind, give it the single purpose of spreading itself, while at the same time, using the host to fuel it and keep it alive.
Let's start with blood. Everyone knows that if there are zombies, there will be a lot of blood. In every movie, every game, there has always been an excess of blood, usually more than is realistic. It wouldn't be because zombies have more blood, no. That just doesn't make sense. The blood simply can't clot, or it is greatly watered down. Since zombies are more simplistic creatures, it isn't likely that the blood would be watered down, in any sort. It doesn't need to get anywhere quickly; there are no places that require more oxygen to function. And it would make perfect sense that the blood simply can't clot anymore. If it doesn't clot, more of it escapes. The more blood escapes, the more chances the parasite has of exiting the host and entering a new one.
This being said, the zombie parasite must not be airborne. It travels through liquids, and, if zombie lore has taught us anything, is highly contagious. Upon contact, the parasite can work its way into a body, through cuts, scrapes, perhaps even pores. Anything that gets it to the blood stream.
From inside, it destroys anything that doesn't send an electrical signal. The nervous system needs to be intact, as it will be the way the parasite controls the host body. The parasite begins by liquefying organs, tissues, and slowly, even the muscles and bones. Everything is melted down to a homogeneous gel, used to fuel the parasite and give it the energy necessary to keep the brain running. It isn't as far-fetched as it may seem, actually. The Ebola virus does the same thing to organs and tissues, for the purpose of keeping itself alive. So, what may seem like zombie blood may be, depending on the age of the zombie, be the entirety of the innards of the creature, not just the blood.
The now-liquid organs have already rendered the host dead, yet the energy absorbed from this broken-down mix gives the parasite enough energy to keep the brain, as well as the untouched nervous system, active. From here, it latches on to the motor cortex, taking control of motor skills, and, in most cases, the cerebellum. From here, it can drive the body to attack, maim, injure, and otherwise infect everything in its reach. The frantic drive to assault other humans makes sense when you realize that, eventually, the muscle tissue and bone will be broken down to fuel the zombie, and once this happens, the parasite has little use left for the host. From the second it enters the human, it has a set timer to spread as far as it can, as quickly as it can, before its host disintegrates.
Well, that's about all I can say. If I was more educated in this topic, I could probably keep going, but I'm not. In any case, if I got this far with next to no knowledge of what I'm talking about, then surely, a bio major could piece together enough details to almost create a zombifying parasite to call their own. Perhaps call it the T-Virus. Resident Evil, here we come.