I’ve heard studies that say it’s not true, but I’ve never believed that the dry air in airplanes didn’t cause colds. I fly pretty often, and for the first part of last year I was flying nearly every week, and the entire time I had a killer cold that I couldn’t shake (which turned into bronchitis). I’d get nearly healthy, then fly, and be right back where I’d started. This article seems to find strong evidence that the dry air is the culprit, not so much the airplane. Combine that with high human traffic places like airports, and you’re ripe for disease.