He is remembered as the co-discoverer of the infectious agent of bubonic plague in Hong Kong during an outbreak in 1894, almost simultaneously with Alexandre Yersin.

Kitasato's discoveries on tetanus and von Behring's work on diphtheria lead to the field of serotherapy, which has benefitted many worldwide.

He traveled to Hong Kong in 1894 at the request of the Japanese government during an outbreak of the bubonic plague, and identified a bacterium that he concluded was causing the disease.

[11] In 1911, Kitasato found himself in Manchuria amidst one of their most severe pneumonic plagues studying its prevention,[4] as he believed his purpose was to investigate the science behind public health and provide advancement in that field.

[4] In September 1921, Kitasato founded, together with several medical scientists, the Sekisen Ken-onki Corporation, with the intention of manufacturing the most reliable clinical thermometer possible.

Kitasato also was the first dean of medicine at Keio University, first president of the Japan Medical Association, and served on the House of Peers.

Although Kitasato transitioned from actively studying disease to politics and conferences, he continued improving upon his work on tuberculosis up until his death and made significant contributions in the realm of public health.