A 2004 study published in the Journal of Enviromental Health Reasearch said 20 percent of people questioned after a 2 1/2 hour flight actually came down with a cold within a week.
The research blamed in part "reduced resistance to infection from the usual very dry cabin air and fatigue"
The dry air can dry out the mucous membrane of your nose, making it easier for a virus to enter.
Some companies are capitalizing on that concept with products designed to lubricate your nose.
Some doctors recommend a tiny dab of petroleum jelly just inside the nose to help block organisms but not all agree this is safe or even works.
Your risk of infection is highest if a sick passenger is seated 2 seats beside, behind or in front of you.
That's according to the latest research by the Centers for Disease Control.
So if that happens to you, ask to move.
Some other tips:
