Researchers tracked the spread of methicillin-resistant Staphylococcus aureus (MRSA) during a one-year period in the East of England, and observed evidence for transmission of the bacteria in the community resulting from clinically unrecognized episodes. Contrary to the assumption that most MRSA cases arise from large hospital-acquired (or nosocomial) outbreaks confined to single wards, Francesc Coll et al. observed a transmission pattern that centered around specific individuals who spread the bacteria as they moved through the healthcare system. Surprisingly, the researchers also found evidence that MRSA strains typically thought to be strictly hospital-associated were spreading in the general community. The researchers mapped transmission with unprecedented detail by prospectively identifying 1,465 individuals in a one-year timeframe who had MRSA-positive samples that came from one of three hospitals and 75 general practitioners' offices processed by a routine diagnostic laboratory. They sequenced the genomes of 2,282 MRSA isolates to identify 173 separate clusters of closely related bacteria, then applied epidemiological data (like hospital admissions, ward transfers, and areas of residence) to find links between cases. Beyond the most common MRSA strains in the United Kingdom, the samples included endemic lineages from Taiwan and the United States, an observation that suggests the authors' findings could apply beyond the United Kingdom to other regions around the world, they say.