

INSECTS AND DISEASE 
tuberculosis infection does in man. At certain times and under 
certain conditions, the disease becomes more virulent, the rats die 
in great numbers, and infection spreads to human beings. The 
agent of transmission of the germ from rat to man and from man to 
man remained to be solved; and in 1897 and succeeding years 
evidence accumulated by a number of French, English, and Russian 
investigators began to point more and more strongly toward the flea 

Fig. 29. HABITAT GROUP OF CALIFORNIA GROUND SQUIRRELS 
( Cttellus varitegatus beecheyt) 
American Museum of Natural History 
as the intermediate carrier of the germ. Finally the experiments of 
the Indian Plague Commission rendered this practically certain, for 
they showed that infection did not spread from a sick to a well rat 
even when in intimate contact if fleas were absent, while if they 
were present, the exposed animals quickly came down with the 
disease. In man it is now known that plague may at times (as in 
Manchuria) develop a peculiar ‘“‘pneumonic’’ type in which the 
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