It was stated earlier in the series that all the survivors are already infected with the zombie virus, which is why even someone killed by a gunshot will reanimate if their brain is intact.
However, why do people who are bitten by zombies wind up dying, if they're already infected? What is the new, 100% lethal and incurable thing introduced via the bite, which the survivors don't already have in their blood?
If they're just falling victim to the general filthiness of being bitten by a rotting human, why do they skip straight to amputation, rather than attempting to use alcohol, boiling water, cauterization, or even penicillin (when available). Or, barring all that, just give the injured person lots of water and hope that they pull through on their own?
I'm preferably looking for canon answers from the show, but I'd also be happy with an answer from the comic if the show hasn't addressed the issue.