In a recent Dinosaur Comic, T-Rex put forth a theory that basically stated the conundrical nature of zombie infections. Essentially, as he states it, zombies really want to EAT you. And when they catch you they will try to devour you entire body (ignoring bones I suppose) and are particularly enticed by your brain.
The problem with that is that if they do eat their victims brain, there is no way that the victim would become a zombie. It's an established fact that zombie infection is centered in the brain, thus the only way additional zombies are created are by being bitten but not killed and keeping their brain intact. Also, zombies are reliant on existing muscle tissue and structure, so if someone was even partially eaten there is a great chance that it would at least partially immobilize them. Still this sets the requirement for the zombie population to expand, zombies must be ineffective hunters.
That poses more problems though. By being ineffective hunters, it makes it much less likely for someone to be bitten by a zombie without first spotting it. In this case, after an initial outbreak (being caught of guard and not understanding the nature of the infection), it would probably not be that difficult to eventually exterminate the ineffective zombies.
If they were effective, you would only have a couple zombies who killed lots of people but the infection would not spread rapidly enough to lead to zombie apocalypse.
Anders Sandberg further addresses this issue with his post Zombie Evolutionary Epidemiology. This article basically addresses the evolution of the zombie 'virus.' First, he says, T-Rex is setting black and white (effective/ineffective) cases for the zombie. Sandberg argues that even an ineffective zombie can occasionally be effective and even an effective zombie can occasionally be ineffective, thus you can't create absolutes like T-Rex does.
Second, he posits that there may be evolutionary pressure on the zombies, consequently making certain qualities of the virus heritable. He reasons:
Assuming zombieness is in some sense heritable (why not? nothing about zombies makes sense anyway), there would be an evolution towards reduced virulence (i.e. biting, not killing as often).
Assuming zombieness is in some sense heritable ..., there would be an evolution towards reduced virulence (i.e. biting, not killing as often). This is just as how many non-vector transmitted diseases evolve towards more benign forms where the host is not killed (diseases transmitted by vectors on the other hand have a weaker incentive to become less virulent). So the initial zombies would evolve towards an optimal speed to injure enough people to keep the spreading high, but not be too efficient.He then models how he thinks either population would fare (effective vs. ineffective). The graph below is also from his article.
This model basically says ineffective zombies start out at near zero and effective zombies at nearly one. Zombie efficiency is essentially the likelyhood that a person will be attacked in such a way that it prevents them from becoming a zombie. For this model Sandberg assumed that each 'child' zombie would have a normally distributed change with standard deviation 0.1 efficiency from its 'parent.'
As you can see from the graph, both populations tend toward a 50% effective rate. It's difficult to tell from the second graph which population is which but either way, humanity is totally screwed.
There is further supposition about how the zombie population may change in response to the availability of its prey. It becomes a question about the evolutionary benefits of shuffling vs. fast zombies. It's in this area that I think the analysis gets a little bogged down. I kind of get the feeling that he's saying that fast zombies are more effective than shambling zombies. I'm not sure this is the case (certainly in zombie movies, they both seem equally effective).
Either way it was an interesting article and definitely worth checking out if you're a fan of zombies.
Thanks to Chuckles for the Zombie Survival Guide for helping me to think about the zombie menace in more scientific terms.