I haven't been sick since going paleo more than two years ago. Meanwhile I even dropped my last, "mysterious" allergy that had accompanied me for 20 years. Not even the tiniest of micro-colds. That alone, man, that alone is worth it all.
Theories? I think it was Taubes who explained a few things about Vitamin C uptake versus sugar and I guess by now most of us will be able to guess that a healthy endocrine system has the power to accomplish almost anything in this regard.