286 • The Great Plaguefor a human.^48 A new plague epidemic would await either a bacterial muta-
tion into a more virulent strain (fitting Boghurst’s notion of endemic plague
in pestered places getting out of hand) or the importation of a new, deadly
plague strain (which fits Hodges’ theory of plague coming from Turkey via
Holland).
The relationship of the human hosts to Yersinia pestismust also be consid-
ered. What of the persons who have been so prominent in our story: the
ever-daring Samuel Pepys, the caring physicians Nathaniel Hodges and
William Boghurst, and Symon Patrick, who visited stricken parishioners?
All of these men came close to infective rat fleas during their great plague.
Many nurses, midwives, searchers, and bearers ran even greater risk of being
bitten by infected fleas. Were these women and men blessed with natural im-
munity during the epidemic? What about immunity in populations in the
years following this last Great Plague?
Interesting evidence might shed light on this. One could have acquired
immunity by exposure to a relatively benign microbe that is genetically close
to the plague bacillus. In the eighteenth and nineteenth centuries, officials in
parts of the Middle East reported a chronic form of plague that caused dis-
tressing intestinal sickness and nothing more. Researchers have now identi-
fied a bacterial agent,Yersinia pseudotuberculosis,that closely resembles Yer-
sinia pestisand produces mild intestinal disorders. Although it is transmitted
by contaminated food or water rather than an insect bite, it belongs to the
same genus as Yersinia pestis,with ribosomal RNA (from small structures
called ribosomeswithin the cell transcribed from original DNA) that is 97. 8
percent identical with that of the plague bacillus. It is now believed that Yer-
sinia pseudotuberculosiswas the original form of Yersinia pestis.
In this near-clone of plague, researchers have found similar antigenic
molecules—proteins that cause the body to produce protective antibodies. If
this close cousin infects humans or rats, its antigenic molecules will protect
against plague. It abounds in many European ports and has been widely
identified in France, from which plague retreated after the epidemic at Mar-
seilles in 1720 – 22. Testing of the sewer system of Paris in the last century
showed high levels of this organism.
The plague bacillus has remarkable homogeneity, with little discernible
change since it mutated from Yersinia pseudotuberculosis 1 , 500 to 20 , 000 years
ago, probably in the central Asian plateau. In a fascinating search backward
from what we know currently, Dr. Carniel and her co-investigators have
drawn on the ribosomal RNA of Yersinia pestis,a template of the original par-