In June 1894 Franco-Swiss scientist Alexandre Yersin arrived in Hong Kong and made a brilliant discovery of global significance. He discovered the bacillus that causes which disease, which was rife in the territory?


In 1894, during the epidemic in Hong Kong, the organism that causes plague was isolated independently by two bacteriologists, the Frenchman Alexandre Yersin, working for the Pasteur Institute, and the Japanese Kitasato Shibasaburo, a former associate of Koch. Both men found bacteria in fluid samples taken from plague victims, then injected them into animals and observed that the animals died quickly of plague. Yersin named the new bacillus Pasteurella pestis, after his mentor, but in 1970 the bacterium was renamed Yersinia pestis, in honour of Yersin himself.

It remained to be determined how the bacillus infected humans. It had long been noticed in many epidemic areas that unusual deaths among rats preceded outbreaks of plague among humans, and this link was particularly noted in the outbreaks in India and China. The relationship was so striking that in 1897 Japanese physician Ogata Masanori described an outbreak on Formosa as “ratpest” and showed that rat fleas carried the plague bacillus. The following year Paul-Louis Simond, a French researcher sent by the Pasteur Institute to India, announced the results of experiments demonstrating that Oriental rat fleas (Xenopsylla cheopis) carried the plague bacillus between rats. It was then demonstrated definitively that rat fleas would infest humans and transmit plague through their bites. With that, massive rat-proofing measures were instituted worldwide in maritime vessels and port facilities, and insecticides were used in areas where plague had broken out. Beginning in the 1930s, sulfa drugs and then antibiotics such as streptomycin gave doctors a very effective means of attacking the plague bacillus directly.

The effectiveness of these measures is told in the declining numbers of plague deaths over the following decades. From a maximum of more than one million in 1907, deaths dropped to approximately 170,000 per year in 1919–28, 92,000 in 1929–38, 22,000 in 1939–48, and 4,600 in 1949–53. Plague is no longer an epidemic disease of port cities. It is now mainly of campestral or sylvatic (that is, open-field or woodland) origin, striking individuals and occasionally breaking out in villages and rural areas where Yersinia is kept in a constant natural reservoir by various types of rodents, including ground squirrels, voles, and field mice. Some 1,000 to 3,000 people worldwide contract plague each year, and some 200 of them die. The main regions of plague are in western North America; the Andes region and Brazil in South America; a broad band across Southwest, Central, and Southeast Asia; and eastern Africa. Most cases today occur in Africa.