Hideyo Noguchi. Portrait of Seisako Hideyo Noguchi (1876-1928), Japanese bacteriologist. Noguchi isolated the spirochaete bacterium Treponema pallidum which causes syphilis. This allowed Noguchi to develop a diagnostic test for the disease. He also discovered that T. pallidum invades the nervous system as the disease progresses. Noguchi investigated Oroya fever and found that the bacterium Bartonella bacilliformis, transmitted by sand flies, caused it. He carried out research into yellow fever and went on a trip to Africa in order to confirm that it was caused by a virus. Shortly before he was due to return home, Noguchi contracted the disease and died.