my theory is that they aren't immune, but resistant. Remember when Tanya said that they had a case of prolonged turning in the colony? how do we know she didn't change some details around...how do you know it wasn't her and she didn't fight it off with the natural resistance plus the drug cocktail she gave saul?
it would make sense as to why saul was ok in ground zero too, the theory may be the drugs, but i have a feeling that it is the fact that he is resistant