For this specific question, it might be worth studying people (especially young people) who get fewer or no colds. It's young people because I have heard that as you get older, your immune system develops a better library of cold viruses to fight off. This matches my experience, though I'll also note that I get the same cold (or at least colds with the same symptoms) a number of times.

In general, I don't think healthy people get studied enough.