TODAY’S biodiversity crisis is not just one of lost habitats and extinct species. It is also a crisis of emerging infectious diseases (EIDs), such as HIV in humans, Ebola in humans and gorillas, west Nile virus and avian influenza in humans and birds, chytrid fungi in amphibians, and distemper in sea lions. There is every reason to take these events seriously, because EIDs appear to have a long evolutionary history.