Underlying genetic variation in the immune systems of rabbits allowed them to rapidly evolve genetic resistance to the myxoma virus, a deadly rabbit pathogen introduced into Europe and Australia during the 1950s, according to a new study. Myxoma virus is a naturally circulating pathogen in American cottontail rabbits, which generally results in benign tumors on the skin. In European rabbits, however, the virus is lethal, causing the systemic disease myxomatosis. During the 1950s, the myxoma virus (MYXV) was released in Europe and Australia as an attempt to curb the rapidly growing and damaging rabbit populations. The release of the virus decimated rabbit populations throughout the region, but it also spurred a rapid evolution of resistance to the virus. However, according to the authors, despite focused research on the genetics of the rabbit-borne disease, little is known about the genetic basis of this resistance. To address this unknown, Joel Alves and colleagues compared the exomes of modern rabbits with those of historical specimens who lived before or shortly after the MYXV pandemic from the countries where genetic resistance to the virus independently emerged; in total, Alves et al. sequenced 152 rabbits from Australia, France and the U.K. The results reveal a strong pattern of parallel evolution; many of the same gene variants were more highly expressed, or otherwise expressed differently, in rabbits across all three countries. Furthermore, many of the changes occurred in immunity-related genes and indicated a multi-gene basis for resistance, according to the authors. They say the reason that parallel evolution of resistance occurred so similarly and quickly in each of the populations was likely due to natural selection on standing variation present in an ancestral population - genetic variation that was retained in the rabbits who colonized the U.K. and Australia.