They have already said that everyone is infected. If you die from anything you turn unless your brain is damaged. They have also said that the bites/scratches cause an infection which kills you.
Yeah, I don't think there is an actual infectious agent delivered by bite or scratch from a zombie.
What happens is the decaying zombie is riddled with nasty bacteria, so basically any contact from them that breaks the skin is going to always result in some nasty staph or similar infections. If that infection is allowed to spread through the body, sepsis will kill you pretty damn quick. Amputation works because that prevents if the infection has taken serious hold yet, and thus hasn't reached the bloodstream, then that's that.
The writers may NOT be working on the principal of bacterial infection and are actively working under the principal of a specific zombie infection agent, but for all intents and purposes, it appears to be one in the same. Whether the characters acknowledge it is a specific infectious/virulent agent, or bites/scratches is simply a perfect delivery for potential sepsis, doesn't seem clear. They've assumed actual zombification agent/infection before, but I don't know how they view it now.
Biologically, there shouldn't be any zombie-specific infectious agent delivered via a scratch, so that seems to support the idea that instances of zombie attack are simply a perfect means of delivering nasty infections that lead to sepsis.
If they could get to a hospital and treat the sepsis, if we are working under this theory, then someone could theoretically survive a perfect zombie bite. Even more likely--as surviving full-blown sepsis, even with first-world medical treatment, has quite low odds--is the idea that medical intervention could prevent a simply localized infection from scaling into sepsis. Treat a bite wound properly from the start, likely removing tissue, full-course antibiotics, skin grafts, the whole works, and that person should survive.
Of course, it's also all fictional work (the comic, the show, the entire concept of zombies), so it really is a moot point.