When flying as flight crew for many years I never got unduly sick (as Tightslot stated) however since starting to fly as pax I would say I pick up a cold/throat bug or sinus infection about every other return flight so say about 25% chance on each sector I fly (and rarely/never get these conditions under normal circumstance)...I can only assume that as flight deck I was either not as exposed to the germs and/or the constant exposure gave me some form of immunity - cant explain it either - but what I do know is being exposed to the recirculated germs of 200 other people in a confined space directly relates to the frequency of these bugs with me.