On June 13, 2011, paramedics rushed a gravely ill 43-year-old woman straight to intensive care. She had a rare lung disease and was gasping for breath. The hospital employed the most stringent and severe form of isolation, but soon the bacterium, Klebsiella pneumoniae, was spreading through the hospital. Seventeen patients got it, and six of them died. Had they been infected by the woman? And, if so, how did the bacteria escape strict controls in one of the nation’s most sophisticated hospitals, the Clinical Center of the National Institutes of Health in Bethesda, Md.? What followed was a medical detective story that involved the rare use of rapid genetic sequencing to map the entire genome of a bacterium as it spread.