About 75% of emerging infectious diseases are now zoonotic, with continued climate change expected to exacerbate transmission in environments shared among plants, human animals, and domestic, agricultural, and wild nonhuman animals. One Health is an approach that promotes national and international collaboration and coordinated responses to human population growth, agricultural encroachment and deforestation, ecosystem disruption, and interactions that intensify human-nonhuman animal transmission risks. Climate change diminishes biodiversity; diminished biodiversity undermines natural ecological balance dynamics between pathogens and hosts and has already influenced health care dramatically. This theme issue investigates the nature and scope of our collective responses to key changes.