Roughly two-thirds of emerging infectious diseases — including COVID-19 and almost all recent epidemics — originate in the bodies of animals. Microbes have spilled over from animals to humans for time immemorial, but, as our species dominates the biosphere and transforms the frequency and nature of human-animal interactions, the rate at which microbes are jumping the species barrier is rapidly accelerating. In this episode, we speak with investigative journalist Sonia Shah, author of “Pandemic: Tracking Contagions from Cholera to Ebola and Beyond,” about the history of viral infections and how our treatment of animals and the planet — via the burning of fossil fuels, biodiversity loss, deforestation, factory farming, the wildlife trade, and more — is fueling the eruption and spread of infectious diseases.