I've had my own silly little explanation of it for a while...
I thought it was a sort of parasite that takes control of the human brain, using the body to seek out food(flesh) for nutrients or what-have-you to nourish itself. On it's own, it's not strong enough to forcefully overtake the brain when it's already in use(living hosts) but can freely utilize it when the host has died.
As for the bite, i never really thought that part out much, but figured it somehow gave the parasite an "adrenaline boost" of sorts so it becomes strong enough to steal control of the brain from the living host, thus causing death.
Well...i was never was the best at coming up with this kind of stuff.