American pathologist and bacteriologist who isolated (1899) a common strain (Shigella dysenteriae) of dysentery bacillus and developed a curative serum for cerebrospinal meningitis (1907).

Simon Flexner was the brother of the educator Abraham Flexner. After teaching at Johns Hopkins University, Baltimore (1895–99), and at the University of Pennsylvania (1899–1903), he helped organize the Rockefeller Institute for Medical Research (now Rockefeller University; 1903), New York City, which, under his leadership, became the world's leading centre for virus disease research.

As director of the Rockefeller laboratories (1903–35) and institute (1920–35), Flexner led the research team that was able to produce in monkeys a nonfatal form of poliomyelitis and pass it from monkey to monkey, thus trapping a polio virus for laboratory study.