Lots of water to stay hydrated. Preferably bottled and in flight I order seltzer to insure it's fresh. (Never trust unsealed bottles). Carry and use True Lemon to add vitamin C. Or add Emergen-C packets to bottled water.

Hand sanitizer wipes are good - often the tray table is the dirtiest item on the plane.

Use the the air vent to direct germs away from you (after using sani-wipe on it)

Generally stay healthy and support your immune system despite pressures of travel schedule - eat healthy, exercise, get 8 hours sleep. Use noise canceling headset to avoid stress of noise in flight. Meditation helps reduce inflammation which makes you more susceptible to illness.

Get an annual flu shot. This year's strain is wicked. Even though the shots don't always protect they do seem to lessen the impact. You might also speak to your doctor about the pneumonia shot if you are prone to upper respiratory infections.

And in act of compassion, offer the person hacking up a lung any cough drops you may have ... if they cough less they may spread less germs. Win-Win.