My theory: the virus is airborne and all have inhaled it. It incubates in the body and upon lack of oxygen/death it activates.
Alternatively, the virus may activate even while the person is alive, but it may take longer, whereas lack of oxygen/death accelerates it.
The reason I say this is because Shane was acting weird prior to his death. Staring off in the distance like a mouth breathing retard, slapping his head repeatedly, etc. Granted, Shane's been becoming unhinged for some time now, but the last episode was pretty extreme. There were points where Shane was excessively drooling for no apparent reason.
With that being said, if Shane was infected and the virus does turn the living at a slower rate, then why aren't the others showing signs? Maybe they're immune? Or the time frame in which a living person will turn is dependent upon that person's genetic predispositions, metabolism, etc.?
Part of me says that the dormancy period theory is more likely, but the one thing that gets me is that if a living person is bitten, they turn. Technically, I guess they die first and then turn, but still, the virus doesn't remain dormant in their system after being bitten until a later point at which they die. So what would be the difference between inhaling the virus and contracting it via a bite? Why does a living person who is bitten die and then turn, but a living person who has supposedly inhaled it does not turn until they die from something else?