With the release of 28 Days Later in 2002, director Danny Boyle introduced a new type of zombie into the popular culture. This type of zombie was no longer a slow and plodding creature, but was fast and agile. They were able to run and jump, tackling their prey like an NFL linebacker. Boyle has said that this vision of the zombie was created to restore some of the terror back to the genre after the comedy Shaun of the Dead had shown just how easy it was to simply outwalk a zombie. That makes a lot of since from a filmmaker's perspective, but it invites the question as to how theoretically probable it is. Given what we know about the human body and how it responds to infections, I would say the probability of a fast moving zombie is low. As Simon Pegg has pointed out, it's hard to run when you have a cold, let alone when you've got a virus so powerful it's essentially killed you. One of the traits we look at for zombies is that their brains have been attacked. Whether this be from a virus, a prion or even some sort of parasite, the effect is the same: the person acts as an emotionless killing machine because the brain is damaged. Because the brain also controls motor skills, we can assume that these abilities would be similarly damaged. Furthermore, decomposition or symptoms like ocular hemorrhaging (experienced as symptoms of some viral infections) would reduce the zombies vision, making it difficult to move at any great rate of speed. If anything, a zombie might begin “fast” (being no faster than the original human host form) but lose speed as its brain and body slowly succumb to whatever is the root cause of the zombification.