If you think about all of the flesh that the survivors come in contact and the splattering, will any of the survivors become immune. (season 3 spoiler below)
I am thinking specifically of Hershel with the ugliest contact viewed in Season 3 (so far).
Could he have developed an immunity to the pathogen?
No, no one is immune. In an interview with Kirkman (I cannot find a current link to the interview, but it is cited in multiple places), he established that:
The rule is: WHATEVER it is that causes the zombies, is something everyone already has. If you stub your toe, get an infection and die, you turn into a zombie, UNLESS your brain is damaged. If someone shoots you in the head and you die, you’re dead. A zombie bite kills you because of infection, or blood loss, not because of the zombie “virus.”
Note that it probably isn't Walker blood or bites that kill people. Instead, it seems likely that death is caused by secondary infection. In fact, there was a blog on scientificamerica.com that did a good job of discussing this.
So everyone has the pathogen that causes them to rise as Walkers upon death, and the risk of fatality-by-bite or fatality-by-scratch is most likely caused by a multitude of secondary infections caused by the many pathogens festering in the mouths and under the finger nails of Walkers. This means that an immunity to the Walker pathogen would go completely unnoticed until the time of death (and we haven't seen this happen yet), and also that no one would be immune to death from the "splattering", since that would require immunity to a host of different things simultaneously.
I actually believe it is possible but extrmemly unlikely.
By this I mean that we as viewers might never see these immunes, however several times throughout the series, Rick and others have passed corpses that have not been reanimated and seem to very much so have their "head/brain" intact.
Actually this puzzled me from early on throughout the series, so in theory I think it is a possibility.
Also we do not know too much about the rest of the continent, nor the rest of the world, here we can assume a world-wide pandemic, however it also increases the chances of a frea immune, a working lab able to develop some form of vaccine etc. After all out of the 6 or so billion People I would not be surprised if the number of living are in the millions, maybe even tens of millions, again incesing the odds of an immune and or some form of resistance or medical vaccine.
