[AnotherUnixGeek]

The low humidity environment will keep some things down.

What I've always heard and read is that the low humidity, by drying out the nasal and sinus passages, lowers our natural protection against airborne viruses. Combine that with a large number of people sitting in close proximity with each other and airplanes seem to be an ideal place to catch someone else's cold.

[TalonDJ]

Yup. That is also true. It trades off with keeping bacteria and molds down.