For your consideration, a guest post from listener Daniel explaining an extensive theory of how zombieism could work in the Walking Dead universe:

Hello Jason and Karen,

My wife and I thought we should offer our theory about zombieism in TWD as a whole.  We have been discussing, over lunch no less, the virus itself and how the transmission could occur.  As to the virus genesis, we initially thought government conspiracy but that’s been done, to death (pun intended).  Then we thought about a corporate cause, that’s been done too and has become a rather lucrative series for Capcom in Resident Evil.  This left us with one more option: Natural Selection.

It’s been established (TS-19) that the virus attacks the brain.  Perhaps, within human DNA, specifically DNA coded to be brain cells, there is a dormant gene that is affected by the virus attack.  This theory would also explain why we have not seen zombie wildlife, no dormant zombie DNA.  Upon depositing its RNA/DNA into the host cell it activates the dormant gene causing it to mutate into a zombified cell, replicate more zombie virus prions and also send conflicting electric impulses throughout the body.  This would, in turn, short out the electrical system of the body possibly causing premature apoptosis or necrotization of major organs and unaffected brain cells as infection worsens.  The fever would either be an effect of the virus attacking the hypothalamus or the lymphatic system’s attempt to supercharge the creation of white blood cells to attack the disease.  As there is evidence (Jim) that those ill will shiver, convulse, and appear to be in a high state of agitation (epinephrine) it’s likely that the hypothalamus is involved.

After brain death, the virus continues to replicate and expand in the brain stem taking over the cerebellum and lower brain functions.  Once there are enough cells under the zombie virus’ control, the electrical impulses become stronger, shock the body back into “undeath,” and reestablish a rudimentary communication system between the brain and the body.  It is a flawed communication as cell necrotization is still occurring in the rest of the body.  The communication is primarily for the neuromuscular system as the rest of the body would be dead, thus slowing atrophy of those cells, since after all, they are still receiving commands.  This would not prevent damage to the fibers though that would occur due to use and since there is no processing of protein there is no cellular regeneration.

The question remains which portions of the brain come under zombie control and which remain dead.  Parts of the temporal lobe (needed for hearing are activated), occipital lobe (needed for vision), and cerebrum (primarily the parts of the subcortical area) are reactivated.  The parietal lobe seems to remain dead (unless the undead are doing algebra off-screen).  Since only portions of the different encephalon layers are reactivated that would provide credence to the issues that the zombies are having coordinating themselves as many of the different portions of the brain require other portions to relay signals.  The neural bridges linking the two halves are most likely also offline so the communication is only going down and not up or left to right.  Thus the failure of processing “hey, I just got pulled in half by my dinner.  That hurt.”

Now for transmission.  There has been a lot of willingness to walk around with open wounds, other people’s blood on the body and clothing, and getting splattered in the face with zombie fluids.  We have concluded that perhaps the only transmission of the virus is through saliva.  We have not yet seen zombification through anything other than a bite.  I believe this theory breaks from traditional zombie tropes but if others are changing popular tropes in other well established horror monsters for the purposes of story-telling, the same may apply here.  This would also go far to explain why there are piles of dead people, not shot in the head, about that are have not been zombified, as in the scene on the highway.

Earlier I stated Natural Selection.  This theory is stemmed from Jenner calling this our extinction event.  After all, it has been alluded to that humanity is a virus (The Matrix), what better way to kill off a virus than with another virus.  We have seen attempts of human population control by nature, using this method, in recent memory with SARS, HIV, H1N1, and Ebola, but they have not had the same impact level as this virus has.  They have succeeded, and my sympathies go to those who have been affected by these virus, in taking human lives but they have been either too short-lived or had weak transmission rates.   I believe the zombie virus was designed to have maximum impact over a short period of time but to not be completely destructive to humanity since we are still needed in the food chain as a predator.  I hold out that if the survivors can last long enough, the zombified humans will end up ‘dying’ again do to cell necrotization and the remaining humanity can slowly begin to reestablish a society.  It will probably take several centuries to some form of remote normalcy and most likely we are looking at a time period very similar to the Dark Ages in Europe.  But society will reemerge.

I now close with a haiku.

Necrotizing death
From virus in saliva
Let’s rethink first base

Daniel