It's really pretty simple:
The show established that everyone has the virus, so when you die you turn unless you brain is injured.
It would make sense that a highly pandemic air born virus got out, it stays dormant until you die. People who were already dead (in graves etc..) can not get the virus because they could not breath (obviously), thus you have to be alive to get the virus in the first place.
After the air born infection, as people started to die of 'normal' causes (accidents, nature death etc..), they started to reanimate. Since the virus was now in an active state when they bit other humans, it activated the bite victims dormant virus. The active virus in a living human creates fever and burns them out until they die. Then, minutes to hours later the virus is active in a normal dead host (as it was probably designed that way) active in only there brain stem. With no higher brain functions the zombie acts only on instinct.
Zombies don't breath, btw
So a dormant virus in contact with an active virus, it will pre-maturely activate the victim's dormant virus, when the virus is active (in a living person) it clashes with living brain functions (since it's trying to run its own functions), thus overloading the brain burning it out. The brain loses the battle and the virus now has a place to run in.
What's scary is, I can totally see how scientists can make something similar, the trick is how does the virus stay fueled to run, that is where you have to have a little suspension of disbelief.
That's an interesting and plausible theory to explain the spread of the virus.
However saying that accepting the notion that zombies do not have to breath or drink with any ill effect - that the zombie virus doesn't replenish the energy of or apparently have means to heal the biological entities they have taken over and yet they survive and thrive . . . requires a "little" suspension of disbelief is like saying that the temperature in the artic is a "little" cool.
The fact that Rick somehow survived in a hospital bed for days or weeks without any nourishment or resources (IVs last for at most 24 hours) and was (all in all) in pretty good shape is a "little" suspension of disbelief. Accepting that Zombies no longer follow established science or physics is a "huge" suspension of disbelief. Its more along the lines of "magic" than science and no one theory can explain it. If Zombie victims really do "die" in terms of the natural functions that humans need to do to survive then its impossible to theorize their existence based on the rules of the universe as we understand them.
For example, if the zombies really only use the small portion of the brain that we were told in the CDC episodes, half of the head tramua attacks we see in the battles shouldn't really stop the zombie target.