In 1913, Japanese immunologist and bacteriologist Hideyo Noguchi discovered that Treponema pallidum (syphilitic spirochete) was the cause of syphilis.

In 1916, following the success of his research on syphilis, he became a researcher in the U.S. with the goal of finding a possible vaccine for yellow fever.  Noguchi became a research assistant to Simon Flexner in 1920 at the University of Pennsylvania, and later at the Rockefeller Institute for Medical Research.

While conducting yellow fever research in Africa in 1928, he contracted the disease and shortly thereafter succumbed to his infection.