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.
A study found that registering outcomes meant that positive outcomes dropped a lot. The researchers looked at 30 large National Heart Lung, and Blood Institute (NHLBI) funded trials between 1970 and 2000. Of those studies, 17 or 57% showed a significant positive result. They then compared that to 25 similar studies published between 2000 and 2012. Of those, only 2 or 8% were positive. That is a significant drop – from 57% to 8% positive studies."
I also ran across a study which had same intriguing, plausible, nuanced results about the effects of timeouts, reasoning, and compromising on improving children's behavior. How much should I trust it?