There is now an authoritative answer on this from Robert Kirkman, creator and current creative head on the comics and TV show in the latest letter column in the comics.
Everyone is infected with whatever the zombie agent is, they turn on death. Bites kill due to sepsis, thats it they do not transfer any zombie agent.
WARNING there is a spoiler in another issue discussed that spoils a cool twist in the comics and I assume TV show, view at own risk.
Ahhhhh. So a properly treated zombie bite isn’t necessarily fatal? As opposed to in Romero’s movies, where they are inevitably fatal within three days…
Too bad for Hershel.
Good find.
*So what does a bite do?
Well… bites, and direct to blood contact with zombie gunk, like the Saviors attacked with, causes death. It’s a strong infection that leads to fever that kills someone. Then the “virus” or whatever is already in them… turns them into a zombie.
I believe Kirkman’s reference to zombie bites confirms the Komodo Dragon theory. However, the reference to “blood contact with zombie gunk” doesn’t play out, at least with the TV storyline, due to the fact that most of the remaining humans would have scratches and cuts from dealing with everyday life, as well as from physical confrontations with zombies. If “zombie gunk” killed, everyone who came into contact with “zombie gunk” would already be dead.
Those born after the original spread of the zombie virus (Rick and Lori’s daughter Judith) may not have the original zombie virus, or may have been born with an immunity to the original zombie virus. Or not. 
I’m of the opinion (fan wank) that zombie bites are a red herring, nothing to do with whatever causes reanimation. Zombies just have really nasty, germ infected mouths, so of course if you get bit you get really fucking sick and die. Especially given the piss poor state of medicine in the post zompocalyptic world. Maybe if there was a way to really properly treat bite victims they might not die, but no one has access to proper treatment, let along the luxury of testing and figuring out what exactly proper treatment might entail. Super antibiotics? Antivirals? Voodoo ritual? Yeah, no one has time to try that shit.
You reanimate for reasons entirely unrelated to the bite. Just like everyone else who dies of anything but head trauma.
You mean exactly what I wrote back in post 13, and extensively discussed and confirmed in the rest of the thread?
Could be. Sort of like how the plague can be bubonic (spread by rats) or pneumonic (spread by droplets).
In the truth-is-stranger-than-fiction department, I was just reading this article in the Atlantic about parasitic wasps:
Apparently, it’s not terrible enough that the wasps lay their eggs in a still living caterpillar, which then hatch and eat it alive. They also inject a viral payload that suppresses the caterpillar’s immune system and tweaks its metabolism (Thanks, nature). However:
I’m now imagining zombies manufacturing a different, non airborne form of virus that just kills you when bitten. (Yes, I read the post from Word of God saying it’s Zombie Gunk)
FYI - “Bubonic” refers to the enlarged lymph nodes associated with specific diseases, and not to which animal spread the disease.
*bubonic
[bu-bon´ik]
characterized by or pertaining to buboes;
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.*
*bubo
[bu´bo] (pl. bu´boes)
a tender, enlarged, and inflamed lymph node, particularly in the axilla or groin, resulting from absorption of infective material and occurring in various diseases, such as lymphogranuloma venereum, plague, syphilis, gonorrhea, chancroid, and tuberculosis.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.*
That can be confusing indeed. Pneumonic, infection of the lungs, can be spread from person to person. The other form requires a vector, such as fleas from rats, marmosets, hamsters, etc.
From wikipedia: "Pathology and transmission
Pneumonic plague can be caused in two ways: primary, which results from the inhalation of aerosolised plague bacteria, or secondary, when septicaemic plague spreads into lung tissue from the bloodstream. Pneumonic plague is not exclusively vector-borne like bubonic plague; instead it can be spread from person to person. There have been cases of pneumonic plague resulting from the dissection or handling of contaminated animal tissue. This is one type of the plague formerly known as the Black Death.[2]"