I've wondered the same thing lately. I've been on mostly paleo (except some raw milk, heavy cream, keifer) since early summer and I haven't been sick since last winter. It's unheard of in my life. By this time of the year, with the weather changes and bouts of heavy winds in Southern California, and exposure to other sick people at work, I should be getting colds, hay fever, and so on. Nada.
Also as I go through various combinations of stress, less than optimal sleep, coffee, some alcohol (wine and tequila mostly) that normally guarantees I'd start feeling weak and down and eventually come down with something. Nothing. I'm the only one I know who isn't wearing a coat yet because the cold and the wind is way more bearable (I'm even just wearing t-shirts outside at night). I did workouts and sprints out on the windiest day lately. No flare ups or congestions. It's a little spooky. But then I remember reading about the Powhatan Indians who would pride themselves on going mostly clothes-free even in the winter months. So it can be done.
The only near-illness event was when I woke up one morning with the signs of needing to vomit. The strong tingly feeling in the back of the mouth, upset stomach feeling, and sudden feeling of heat coming from the head. I wander to the bathroom thinking it must be food poisoning or something. But by the time I'm standing over the toilet, it all goes away as quickly as it came. And that was it. Never had that experience before. When the signs are there, the vomit would always follow. What's up with that?