Assuming this is a bio-engineered virus, I'd assume there are two. You infect a population that will have a higher than average rate of death - let's say at a hospital - with both virus' at once. Neither virus 'activates' until death. One re-animates the dead. The other is lethal to living humans. Of course, it could be one virus that does both, but I lean toward two, where each one is very specifically engineered to do a single task.
Since everyone is infected, and the walker virus lays dormant until the host dies, what about the virus that kills? Is that effect/second virus also dormant? Or when Rick bit the guy in the throat, had he lived, would he have died from the bite?
