A deadly, drug-resistant superbug outbreak that began last summer at the National Institutes of Health Clinical Center claimed its seventh victim Sept. 7, when a seriously ill boy from Minnesota succumbed to a bloodstream infection, officials said Friday. The boy was the 19th patient at the research hospital to contract an antibiotic-resistant strain of the bacterium Klebsiella pneumoniae that arrived in August 2011 with a New York woman who needed a lung transplant. But his case marked the first new infection of this superbug at NIH since January — a worrisome signal that the bug persists inside the huge brick-and-glass federal facility in Bethesda.