Every day the average person draws in 2,000 gallons of air—and with it, thousands of living things. The bacteria, fungi, algae, and other organisms that inhabit the air constitute an invisible ecosystem known as the aerobiome, one of the last great frontiers of science. New York Times columnist and author Carl Zimmer draws on his new book, Air-Borne, to explore how the aerobiome is an essential part of a living world—a highway through the sky for many species—of which we become a part with every breath we take.