While it is certainly possible you got sick on the airplane itself, don't factor out the trip to the airport (cab/public transit) and all the different crowds your have to deal with at the airport itself. I know airport staff do the best they can, but during particularly busy travel times airport bathrooms can be incredibly disgusting in my experience.
Aye, it's far from certain it's happening on the aeroplane itself, but I've assumed it is because the air is so dry, and because I was always travelling in business, using lounges with nice toilets, not being next to other people outside of the plane etc.
