While it may be possible that their is an increase in colds after flights, it's probably down to a combination of factors - the cold, dry air in planes makes th

While it may be possible that their is an increase in colds after flights, it's probably down to a combination of factors - the cold, dry air in planes makes the ideal conditions for cold viruses in your nose and sinus, the shared bathrooms and confined seating makes it easy to pick up the virus on common surfaces, and finally, the time period before flights is often a time of stress (getting ready, making it to the airport etc), and the increased cortisol suppresses the immune system. All that makes a perfect setup for a cold virus to take hold. I would guess that taking a set of stressed people, and making them sit in a confined space with cold, dry air for ventilation would result in an increased incidence of viruses even with a perfect ventilation system.