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The first director of the Rockefeller Institute for Medical Research, where from 1901 to 1935 he built an American center for laboratory medicine essentially from scratch. His research on poliomyelitis and his development of a serum treatment for meningitis saved lives before modern vaccines existed; the bacterium Shigella flexneri and the Flexner-Wintersteiner rosettes seen in retinoblastoma both carry his name.