From The Citizen Scientist:
Martin Hocking and Harold Foster of Canada’s University of Victoria have studied the problem of increased colds among airline passengers. In an article for the Journal of Environmental Health Research (“Common cold transmission in commercial aircraft: Industry and passenger implications,” 2004) , they reported that 20 percent of passengers who flew on a 2.5 hour flight developed colds within a week.
Depending on three different flight scenarios, Hocking and Foster found that airline passengers in three different scenarios were 5, 23, or 113 times more likely to catch a cold than if they had not flown at all!
The scientists also found that the threat of catching tuberculosis is substantially higher if an infected passenger is aboard a flight.
The most logical reason for infections would seem to be the limited amount of cabin air shared by the passengers. But Hocking, Foster and other scientists have found this is only one factor. The very low humidity in an airplane seems to be much more important.
There’s more.