I am sure having re-circulation systems is a big bonus if you want to be exposed to every bug known to man;de-hydration, fatigue etc must also be a big help to the immune system.To add to this, in "certain" companies everyone is "encouraged" to go sick as little as possible,resulting in 10 people going sick because they sat next to a sniffling wreck for 4 sectors, rather than one person, wisely, going off sick for a coupla days; really bright on the company's part that.Personally I find if I catch a cold coming up to a week of early starts I'm stuck with it, on late's I might shake it off. I recently tried a homeopathic product "oscillococcinum" (try saying that with a cold) which is marketed (in France at least) by Boiron,and have, so far, found if you can start the treatment just as you are starting to get the very first feeling of getting a cold that it actually works,but then again so might a Glenmorangie,or indeed a good nights sleep. The first winter I worked for Ryanair I was constantly sick (and "of course "still flying).One of the old lags explained to me it was just my body acclimatising to a "different class" of bug;he may have had something there ,every year since I have been OK!