It is most likely airborne, yes. And Hershel was bitten and lived. They cut off his leg soon after he was bitten and he was fine. If the bite spread an "active" virus, shouldn't that then activate the "dormant" virus already in the host? Being able to amputate bitten extremities and not have the fever and not turn seems to suggest that whatever causes the fever is different from the actual virus itself.
To clarify.. it's not the fever itself that kills. The fever is a symptom of the body fighting an infection. Similar to the way the people in the prison got fevers from the Flu-like disease that was spreading. Whatever the infection is that comes from bites, there's no known cure, so it can't be survived without amputation. That's why I feel like the bite causes a blood infection of some kind. It spreads really fast but if the infected area is removed from the body before it can spread, the victim seems to be just fine.
Edit: The CDC guy told them enough to let them know they couldn't find a cure and he believed they weren't going to. I think if there were two different strains, he would've mentioned it.
All good points. I suppose it's possible Rick cut off Herschel's leg before the virus could spread and activate in other parts of the body. I think the dormant/active theory still could fit. With your theory it almost seems like there would have to be another virus in play, not just an infection, but something else entirely that would result in the guaranteed death.
It's true that the CDC guy did spend a lot of time studying the virus, but maybe he didn't see the point in sharing that information at that time since he was going to kill them all anyway. If you think about it, knowing that information doesn't really do much good. It's still the same rules, if you get bit you die. If you die, you turn to a zombie. Knowing about a dormant/active state maybe was just something he didn't feel the need to share because of the point he was trying to make that they were all doomed.
You could be completely right of course. I hope we get some answers from Eugene this season, though I doubt he can actually cure anything, he might be able to explain some more about how the virus works.