Has Flying Made You Sick?

This has nothing to do with Real Estate.  I had a full day of flying on Tuesday, and immediately upon my return, I came down with a nasty virus.  I haven't been this sick in 20 years.  Several people have told me that they came down with a similar illness immediately after flying.  My mom believes that the airlines don't concern themselves with the air quality inside planes.  Every flight I took was booked solid.  With so many bodies in such a tight space, it would make sense that air quality would suffer, especially if the filtration systems have been neglected.  Just curious if any of you have had a similar experience with becoming ill right after flying.  Please share!