Two years ago, canine distemper virus (CDV) felled a third of the lions in Tanzania's Serengeti National Park. How did a virus historically restricted to dogs and a few other carnivores suddenly jump to cats? Now a genetic analysis indicates the Serengeti organism is a new variant. Researchers have also been able to trace environmental changes that set the stage for the mutation: human settlements along the park's border that placed CDV-infected domestic dogs cheek-by-jowl with park animals.