Science writer David Quammen explores one result of our "infernal, aboriginal connectedness" to the animal world: a lot of diseases spilling over from other species to us. Examples include SARS, HIV, avian flu, Lyme disease and Ebola. Quammen says the phenomenon is on the increase, thanks to human actions. We discussed the science behind "zoonotic" diseases and David's globe-trotting research for his new book "Spillover: Animal Infections and the Next Human Pandemic."