It seems obvious now that everyone is carrying a dormant version of the virus, that becomes active after the host dies. This would also explain why a bite from a zombie is lethal; the individual is infected with an activated form of the virus.
It is also clear that the activated form of the virus is transmitted via the blood stream since people are constantly splattered with zombie goo and don't become infected (despite the fact people are constantly walking around with open wounds). It's obviously not transferred orally, and doesn't have the ability to permeate through external or internal epithelial tissues, but can pass through the blood/brain barrier to target the brain.
I'm trying to remember if we saw any dead non-walkers in the show, besides the two officers/guards from a few episodes back. I can't recall any. It is also interesting that once the host is dead, the virus takes hold at different rates. Perhaps the longer the dormant version exists within the host, the faster it takes over once the host dies.
Anyway, that's what makes most sense to me.
__________________
|