Mr. Adventure wrote:
So if they're already infected shouldn't a zombie bite not be so fatal?
Even though they are all infected the pathogen strain is passive until the moment of death, then it becomes active. When they are are bit by a zombie, it's active strain passes to the victim thus causing the victim to die. Medically speaking this does happen in nature with various viruses that have dorment/active stages.
Or, people don't die from zombie bites. They die like Amy did - from massive blood loss, and then they turn. Wouldn't have mattered whether a zombie had caused that massive blood loss. If they're already infected, the possibility remains that the virus is simply taking longer to turn them than it did for others. We've already seen that post-mortem, the "incubation period" varies widely.
But they can't have the characters thinking along these lines until the vaccine plotline is close to kicking in. It would render them too hopeless. Shane was starting to turn into a zombie before Rick killed him. That was an ironic stroke of luck, because if Rick had seen Shane turn without being killed or bit, that would be utterly hopeless for everyone and they'd all just commit suicide, end of story.
The producers can keep dropping little hints about it, but there's a good season or two ahead before they need to activate that plot twist.