He is remembered as the co-discoverer of the infectious agent of bubonic plague in Hong Kong in 1894, almost simultaneously with Alexandre Yersin.
He studied its toxin and its properties, and began in 1891 to develop an effective serum to treat the disease, following the demonstration, by Emil Adolf von Behring (1854–1917) and Kitasato Shibasaburō (1852–1931) that antibodies against the diphtheric toxin could be produced in animals.
Yersinia pestis, the first described species, was identified in 1894 by A.E.J. Yersin, a Swiss bacteriologist, and Kitasato Shibasaburō, a Japanese bacteriologist.
In 1888 Emile Roux and Alexandre Yersin isolated diphtheria toxin, and following the 1890 discovery of an antitoxin-based immunity to diphtheria and tetanus, by Emil Adolf von Behring and Kitasato Shibasaburō, antitoxin became the first major success of modern therapeutic immunology.