The thing about planes is that pretty much everyone wants to get off them as soon as they get on. To deplane is to rejoin a world where you can get up from your seat whenever you want, gulp non-recirculated air, and enjoy more than precisely 10 peanuts at a time. But perhaps the worst thing about air travel—worse than the overpriced snacks and jet-engine noise—is the fact that you're crammed into a confined space and forced to interact with the other people inside it for hours on end.
Planes make us reckon with the fact that humans are gross, virus-carrying, disease-burdened flesh sacs. You can avoid someone coughing in the grocery store, but you can't escape inhaling the same recirculated air as the man in 14B who sounds like he's dislodging part of his lung.
And that's why scientists study disease transmission on aircraft. Three billion people fly every year, and studies have shown that several serious illnesses have hitched rides to areas they never could have reached otherwise. The SARS virus jumped from Hanoi to Paris in just 14 hours and 50 minutes. H1N1 influenza hopped from L.A. to Auckland and Birmingham in under 10. These transmissions would never have been possible without air travel.