. Guide leaflet. unchecked for tenyears and carried off 6,000,000 people. This time, however, theworld invasion of the Black Death was to be met by a new defensivemechanism, the organized force of scientific research. A Japanesebacteriologist, Yersin, discovered the bacillus of plague in 1894 andit was soon proved that the disease from which rats were simultane-ously suffering (as they had done in the days of Samuel) was thesame as the human plague. The infection may be more or lesschronic among the rodents, persisting among them for years as 52 INSECTS AND DISEASE tuberculosis infection does
. Guide leaflet. unchecked for tenyears and carried off 6,000,000 people. This time, however, theworld invasion of the Black Death was to be met by a new defensivemechanism, the organized force of scientific research. A Japanesebacteriologist, Yersin, discovered the bacillus of plague in 1894 andit was soon proved that the disease from which rats were simultane-ously suffering (as they had done in the days of Samuel) was thesame as the human plague. The infection may be more or lesschronic among the rodents, persisting among them for years as 52 INSECTS AND DISEASE tuberculosis infection does in man. At certain times and undercertain conditions, the disease becomes more virulent, the rats diein great numbers, and infection spreads to human beings. Theagent of transmission of the germ from rat to man and from man toman remained to be solved; and in 1897 and succeeding yearsevidence accumulated by a number of French, English, and Russianinvestigators began to point more and more strongly toward the flea. Fig. 29. HABITAT GROUP OF CALIFORNIA GROUND SQUIRRELS ((itelliis Im iegatus beicheyi) American Museum of Natural History as the intermediate carrier of the germ. Finally the experiments ofthe Indian Plague Commission rendered this practically certain, forthey showed that infection did not spread from a sick to a well rateven when in intimate contact if fleas were absent, while if theywere present, the exposed animals quickly came down with thedisease. In man it is now known that plague may at times (as inManchuria) develop a peculiar pneumonic type in which the 53 . XMERh IN Ml SI-J J/ Gl //>/?: LI-:. XFLETS germs are discharged from the mouth and nose as in the case of acommon cold. Pneumonic plague, therefore, spreads directly fromman to man by contact, but in the ordinary or bubonic plague thegerms are not discharged in any of the excretions of the body andcan only be transmitted by the flea. In northern Asia the rodenthost of the plague bacillus is the Siberian
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