Simon Flexner attended the Medical School of the University of Louisville and graduated in 1889 with a M.D. degree. He later studied pathology under Dr. William Welch at Johns Hopkins University. In 1891 Flexner was appointed a Fellow in Pathology, and he continued to be associated with Johns Hopkins University and the Medical School until 1899, during which year he was appointed Professor of Pathology and studied dysentery in the Phillipines. He isolated a bacillus which he demonstrated to be the etiologic agent in many of cases, and the organism has since been known as the Flexner bacillus.

When the Rockefeller Institute for Medical Research opened its first laboratories in 1904, Flexner was named director. He acted in this capacity until his retirement in 1935, after which the title Director Emeritus was bestowed upon him. His research includes infantile paralysis, sleeping sickness, and other infectious diseases.S Dr. Simon Flexner made great advancements to the practice of medicine and pubilc health.