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PRINCIPLES OF BACTERIOLOGY 
tionship to the epidemic disease. The best explanation 
for the recurrence of the epidemics is probably the short 
duration of the immunity conferred by an attack of the 
disease. 
The Bacillus Pertussis 
The bacillus pertussis causes whooping cough, and was 
discovered by Bordet and Gengou in 1900. It is a very 
small, short bacillus, almost resembling a coccus, Gram¬ 
negative, nonmotile, has no flagella, no spores or cap¬ 
sules. It grows on defibrinated blood agar potato. 
Patients with whooping cough should be isolated. The 
complement-fixation test is used in the diagnosis, but is 
not yet perfected; the bacillus produces an endotoxin. 
The immunity treatment—the pertussis vaccine—is of 
great value in shortening the disease and hastening re¬ 
covery. 
Bacillus Pestis 
Bacillus pestis which causes the most fatal of all dis¬ 
eases—the bubonic plague—was discovered by Kitasato 
and Yersin in 1894. It is difficult to grasp the havoc 
wrought by this scourge; in the reign of Justinian one- 
half of the population was wiped out by the plague. 
Known as “Black Death,” it swept the entire world in 
the fourteenth century, killing 25,000,000 people. It is 
always present in India exacting as its toll several thou¬ 
sand lives yearly. Bacillus pestis is a thick, short bacil¬ 
lus, Gram-negative, has no spores, no capsules, no flagella, 
and is not motile. It grows well on meat infusion media. 
It is aerobic, and may remain alive for months and years 
in the dark, if sufficient moisture is present. It lives for 
two weeks in the pus and sputum of patients. Complete 
drying kills the bacilli in two to three days, live steam 
in a few minutes. The disease is usually acquired by 
