foxfly... Most virusses spread via direct contact or contaminated surfaces. Shaking hands is a way of exhanging pathogens or touching something that has been touched by another passenger. Touch your eyes or nose after that and there is a probability of getting an infection.
The population of bacteria in your home is remarkably stable... but you are likely to meet quite some new ones in airports and planes. Washing your hands regularly and using wet (disinfecting) wipes reduces the chance of infection.