A bacterium damaging to the lungs and once thought to be acquired from the environment is actually spread--at least in cystic fibrosis clinics--through human transmission, a new study reveals. The study, which used clinical data from cystic fibrosis centers in Europe, the U.S., and Australia, points to three particularly virulent versions of this bacterium having emerged in recent decades. Mycobacterium abscessus, a species of multidrug resistant mycobacteria, has surfaced as a significant global threat to individuals with cystic fibrosis (CF) and other lung diseases. Previously, it was thought that patients acquired M. abscessus infection from water or soil, and that transmission between patients never occurred. Given the increasing incidence of M. abscessus infections in CF and non-CF populations, however, Josephine Bryant and colleagues sought to better understand whether cross infection, rather than independent environmental acquisition, might be the major infection source. They sequenced whole genomes of over 1,000 versions of M. abscessus from more than 500 individuals attending global CF centers, uncovering near-identical M. abscessus isolates even in different geographies; this suggested the bacterial clones were being widely transmitted within the global CF patient community. Further analysis suggested that the infection may be spread within hospitals via contaminated surfaces and through airborne transmission. How the dominant clones have spread between continents remains unknown, the researchers say; they found no evidence of patients or equipment moving between CF centers in different countries. The study illustrates the power of population-level genomics to uncover modes of transmission of emerging pathogens.