The long-running debate about why just one of several canine distemper virus (CDV) outbreaks in the Serengeti in Tanzania during the past 25 years was fatal for lions and spotted hyenas has been resolved. An international team of scientists conducted genetic analyses of CDV strains obtained from a range of carnivores between 1993 and 2012 and discovered that lethal CDV infections in lions and hyenas during the 1993/1994 epidemic was caused by a rare and genetically distinct CDV strain with three rare mutations not present in any other Serengeti strain isolated from domestic dogs or wild canids. Two of these rare mutations were found to increase the ability of CDV to invade lion cells.