276 • The Great Plaguemass of bacilli, all identical. They are very small rods, thick with rounded
ends and lightly colored.” He categorized them as gram negative and non-
motile. Yersin sent specimens in test tubes to Paris for corroboration and
confirmed his diagnosis when his plague bacilli produced the same signs in
his animals as in infected humans. On August 18 his illustrated report
reached the world through the Pasteur Institute’s Annales.The bacterium,
initially called Pasteurella pestisafter Yersin’s mentor Louis Pasteur, was later
fittingly renamed Yersinia pestisafter its discoverer.
Kitasato’s and Yersin’s dramatic search for the plague bacillus brought
them to the next step of discovery: finding a reservoir where the bacteria
flourished. Both men had observed a large number of dead rats and flies in
Hong Kong. It did not take long to confirm their suspicions that the rats
were involved in the plague epidemic. Drawing blood from dead rats, Yersin
found it packed with plague bacilli. “The rats are certainly the great propaga-
tors of the epidemic,” he concluded.^26 Yersin knew that the flies were also in-
fected but questioned how they could pass on the microorganisms.
The discoverers pondered the route of the bacilli from rat to human, won-
dering how to break the chain of transmission. Kitasato posited three entry
routes of the bacteria to human victims: an external wound, ingestion, and
respiration. Yersin went further in his probing. Discounting ingestion and
respiration, he suspected an insect to be a transmitting carrier. He located an
inoculation site near an infected bubo but did not know the perpetrator.
Epidemiology was just on the threshold of identifying parasitic “vectors” of
communicable diseases.
Five short years after the epidemiological breakthrough at Hong Kong,
Paul-Louis Simond of the Pasteur Institute demonstrated that the rat flea
was a missing biological link in the world of the plague pathogen, with some
fleas infecting rats and other fleas transferring plague from dead rats to hu-
man victims. So Yersin was right in looking for an insect vector contributing
to bubonic plague. But Kitasato was also on the right track in suspecting res-
piration as the human contributor to pneumonic plague through the sneez-
ing, exhalation, and inhalation of infected droplets.
Scientific sleuthing had now uncovered the bacillus that caused modern
plague (and presumably its early modern precursor) and linked the four par-
ticipants whose interaction resulted in the outbreak of plague in human
communities, the famous ménage à quatreof the bacteria, flea, rat, and hu-
man.^27 The challenge now was how to defend the human population against
future plague epidemics. Should the environment be cleaned up or humans
protected more directly by vaccination?