I am not entirely sure of what you're question is but maybe this answer will clarify from a French Wiki page:
Researchers working in Asia during the "Third Pandemic" identified plague vectors and the plague bacillus. In 1894, in Hong Kong, Swiss-born French bacteriologist Alexandre Yersin isolated the responsible bacterium (Yersinia pestis) and determined the common mode of transmission. Japanese physician and researcher Kitasato Shibasaburō initially misidentified the bacterium. In 1898, French researcher Paul-Louis Simond demonstrated the role of fleas as a vector.
The disease is caused by a bacterium usually transmitted by the bite of fleasfrom an infected host, often a black rat. The bacteria are transferred from the blood of infected rats to the rat flea (Xenopsylla cheopsis). The bacillus multiplies in the stomach of the flea, blocking it. When the flea next bites a mammal, the consumed blood is regurgitated along with the bacillus into the bloodstream of the bitten animal. Any serious outbreak of plague in humans is preceded by an outbreak in the rodent population.
During the outbreak, infected fleas that have lost their normal rodent hosts seek other sources of blood. The bacterium that causes this disease, Yersinia pestis, was named after Yersin. His discoveries led in time to modern treatment methods, including insecticides, the use of antibiotics and eventually plague vaccines.
The British colonial government in India pressed medical researcher Waldemar Haffkine to develop a plague vaccine. After three months of persistent work with a limited staff, a form for human trials was ready. On January 10, 1897 Haffkine tested it on himself. After the initial test was reported to the authorities, volunteers at the Byculla jail were used in a control test, all inoculated prisoners survived the epidemics, while seven inmates of the control group died. By the turn of the century, the number of inoculees in India alone reached four million. Haffkine was appointed the Director of the Plague Laboratory (now called Haffkine Institute) in Bombay.
Third plague pandemic