Simon Flexner (March 25, 1863 - May 2, 1946) was an American physician, scientist, administrator, and professor of experimental pathology at the University of Pennsylvania. He pioneered field investigations where infectious diseases were potentially epidemic. Amongst Flexner's most important achievements are studies into poliomyelitis and the development of serum treatment for meningitis. The bacteria species Shigella flexneri was named in recognition of Flexner. In addition, Flexner was the first to describe Flexner-Wintersteiner rosettes, a characteristic finding in retinoblastoma. He discovered the Flexner bacillus, the cause of a common form of dysentery, and the Flexner serum for treating meningitis. His research expertise was already legend when he was selected as the organizing director of the Rockefeller Institute for Medical Research in New York City. For thirty-five years he cultivated the spirit and guided the work of the new institute, while implementing John D. Rockefeller's vision of bringing medicine into the realm of science. He died in 1946 at the age of 83 from a myocardial infarction.