44G THE HEMORRHAGIC SEPTICEMIA GROUP 
very great. No theory has been presented in explanation of this very 
unusual phenomenon, and the origin of the pneumonic type of the 
disease is not definitely known. ^ It is apparently true, however, that 
pneumonic plague occurs in epidemic form only where the climate is 
very cool and damp. Pneumonic plague practically never rages in the 
tropics. Typical pneumonic plague resembles lobar pneumonia in its 
symptomatology, and the fatalities are very great. Death usually 
inter\'enes in less than a week. The marked cardiac depression which 
is a feature of this type of plague is of some differential diagnostic value. 
One or more lobes are infected, the inflammation being catarrhal in 
nature. Enormous numbers of plague bacilli are coughed up with the 
sanguinoserous exudate, which are readily transmitted to doctors, 
nurses and attendants by droplet infection. A>ry frequently a gen- 
eralized invasion of the blood stream occurs. 
Bubonic plague, the most common type of the disease in man, is 
essentially a localization of plague bacilli which have gained entrance 
to the tissues through the skin in the regional lymph glands. The 
inguinal glands are more commonly invaded; next in order of fre- 
quency are the axillary, then the cervical glands. The glands are 
violently inflamed, and not infrequently soften and caseate. A 
generalized blood infection— septicemic plague— may occur either 
secondarily following the development of a bubo, or of pneumonic 
plague, or, less commonly, as an initial generalized invasion following 
very shortly after infection and before the bubo becomes conspicuous. 
In such cases the organisms may be obtained in pure culture from 
the blood stream, and in about 20 per cent of cases may be actu- 
ally demonstrated in stained preparations of the blood by microscopic 
examination. 
Immunity and Immunization.— Recovery from one attack of plague 
in man almost always confers lasting immunity. Immunity has been 
induced in animals — monkeys, rats and guinea-pigs— by inoculation 
with living, avirulent cultures. Usually a moderate reaction is noticed, 
and a bubo may even form, but the animal recovers, and even after 
months successfully resists several times the fatal dose of virulent 
organisms. This method is far too dangerous for himian practice.^ 
Cultures of plague bacilli heated to 50° C, or killed with chemicals, 
as phenol or alcohol, have also been employed successfully, but the 
degree of resistance to subsequent infection is less.^ Specific bac- 
teriolysins and agglutinins develop during the immunizing ])rocess. 
B. pseudotuberculosis rodentium, an organism that occasionally is 
' Animal experiments suggest that attentuated cultures of Bacillus pestis which fail 
to kill guinea-pigs by subcutaneous inoculation may give rise to a fatal infection when 
inoculated into the respiratory tract, and that the virulence of cultures may be recovered 
by this process. The high mortality observed during epidemics of pneumonic plague 
in man may be a similar phenomenon. 
2 Kolle and Strong: Ueber Schutzimpfung des Menschen mit lebenden abgewaschter 
Pestkulturen, Deutsch. med. W^chnschr., 1906, 32, 413. 
3 Kolle and Otto: Ztschr. f. Hyg., 1903, 45, 507. 
