The threat of being bitten.

This has been on my mind .. According to the show, when someone dies of natural causes or otherwise (aside from a walker bite) They will come back as a walker because apparently humans already carry the virus, but if someone is bitten by a walker, this also causes someone to die and then, turn into a walker (even if it was a scratch or minor wound, essentially non-fatal). So my question is, how is a walker bite so threatening if the people already carry the virus anyway ?