Book excerpt: It's not possible for people to walk, or even drive, to many places. Buses, trains, and airplanes get us where we need to go. Unfortunately, wherever large groups of people gather, there are sure to be a lot of germs—and other unpleasant things, too. The more we learn about these things, the more we can keep healthy and adopt behaviors to keep others healthy as well. The gross things hidden from view, or impossible to see with the human eye, on several modes of transportation are discovered through accessible text and engaging, fun facts.