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.