Simon Flexner (1863-1946), an American physician trained at the Louisville (KY) College of Pharmacy and the University of Louisville, where he received an MD in 1889. A student of microscopy in the off-hours, he made microscopic examinations of pathological tissues for doctors who patronized his pharmacy.

In 1890, Flexner arrived at the Johns Hopkins Hospital in Baltimore where he undertook studies of bacteriology and pharmacy under William H. Welch, earning a fellowship and a position as the first assistant in the department of pathology when the hospital opened a medical school in 1892. The following year, Flexner was appointed resident pathologist. Six years later, in 1899, he was promoted to full professor. That same year, Flexner traveled to Manila where he isolated a bacillus, now known as Shigella dysenteriaeis, aka "the Flexner bacillus," as a cause of dysentery.

Flexner then moved to Philadelphia where he had secured a position as professor of pathology at the University of Pennsylvania. Here he made important experimental studies of dysentery, pancreatitis, and immunological processes in the blood. Among Flexner's closest associates at Penn was the Japanese physician Noguchi Hideyo, who later joined him at the Rockefeller Institute.

In 1901, the United States government sent Flexner and some colleagues to San Francisco to assist with an outbreak of bubonic plague. The same year, Flexner was elected to the American Philosophical Society and appointed to the board of scientific directors of the new Rockefeller Institute for Medical Research in New York City. Flexner, who was becoming known as a national expert on disease, was appointed head of the institute's department of pathology and bacteriology and, shortly thereafter, tapped to lead the entire institute. During this time, Flexner also took over the editorship of the Journal of Experimental Medicine, serving as chief editor for the next fifteen years.

Despite his heavy institutional responsibilities, Flexner made additional contributions to the control of disease. To combat a 1906 outbreak of cerebrospinal meningitis, Flexner developed a serum that remained the best treatment until the introduction of sulfa drugs. During the 1910 poliomyelitis outbreak in New York, Flexner and his assistants developed a method of keeping the virus alive for testing in the laboratory using primate-to-primate transfer, a process that was superceded by less expensive cultivation using hens' eggs and led the way to the vaccines of the 1950s.

Before resigning his directorship of the Rockefeller Institute in 1935, Flexner served as chairman of the Public Health Commission of New York State, as medical consultant of the U. S. Army during World War I, and as a member of the China Medical Board of the Rockefeller Foundation.

Flexner was elected to the National Academy of Sciences in 1908. He was made foreign member of the Royal Society in 1919.


Corner, George W. “Simon Flexner.” Complete Dictionary of Scientific Biography. Vol. 5. Detroit: Charles Scribner's Sons, 2008. 39-41.

Last modified 27 January 2017