First of all, most of us over indulged and underslept, so it set’s us up to be more susceptible to catching Colds/Viruses.
In the plane you may have seen on the screen that the air outside the plane is -45 F, so the plane has to heat any of the air it circulates into the plane, this in turn uses fuel, which costs money. So, the refresh rate of the air on the plane is set for as low as is considered reasonable to save fuel. So take a bunch of us, coming off a heavy partying schedule, put us in plane with somewhat stale air and viola, your sick.
I made it back fine. But since i tend to chain smoke while i’m on the road (much more than at home) i have about 25% change of coming home with a chest/sinus infection. This time, i’m not ill, last December, after WebmasterWorld i was sick for 3 weeks.