Carl Zimmer, acclaimed science writer and author of "Airborne: The Hidden History of the Life We Breathe", joins the show to explore the unseen world within the air around us. He talks about what most of us never think about: the air we breathe. He explains how scientists slowly uncovered the truth about airborne diseases, why it took decades for basic facts to gain acceptance, and what history, both recent and ancient, tells us about the hidden life in the atmosphere.