my impression of this is just that the bite and infection gives you a concentrated dose of the virus that accelerates the process and causes you to die once the virus infects you enough. That is the only difference when you get bit IMO. I could be wrong, thats just my impression
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It is actually quite simple, the virus manisfest once you turn and turns into something that can only be spread via the bloodstream and does not actually make you turn. Instead, you die from it and then turn. Hence, why they get a crazy feaver first.