For decades MRSA, or methicillin-resistant Staphylococcus aureus, has been afflicting hospital patients and prison inmates with life-threatening and difficult-to-treat skin infections. In recent years this bacteria has emerged in the general population. According to the Centers for Disease Control and Prevention, it is now causing 19,000 human deaths in the United States per year—more even than HIV. A recent study of the pathogen by researchers from the National Institute of Allergy and Infectious Diseases, part of the National Institutes of Health, reveals how changes in the bacteria’s genome can affect its ability to attack its host.