My personal and unscientific opinion is that this is more the culprit than anything else. Well, this and the fact that most don't wash their hands.

Quite icky the things one sees and hears (and smells!) when sitting on the jumpseat right next to the lavatory - and you guys think your seats are bad!

Seriously though, there has been some limited scientific study of airborne illness and air travel. In a few rare instances when a person with contagious TB is known to have flown, the CDC has gone back and interviewed as many other passengers as possible to assess risk levels. If I recall correctly, it was found that passenger-passenger transmission was virtually nil, passenger-crew transmission extremely rare - but crew-crew transmission was relatively high. Now one or two small studies do not make hard facts, but it's evidence that your risk up there is likely much lower than many people fear.