Jetlagged, reference your quote on this subject:
"many pax actually catch a cold during the flight."
The Common Cold is a virus and one is not able to "catch a cold during the flight" from being cold. The reason so people catch colds while flying is dud to the fact that air in an aircrafts cabin is recycled and recirculated several times. Therefore anyone who has the airborne common cold virus will indeed more than likely pass it on to other passengers.
P.s. I realise this is not a medical forum, just couldn't resist!