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came to regard deaths among rats as foretelling the coming of the 

 plague and fled from their homes, when confronted with this evidence. 

 The discovery of the bacillus and the study of its effects on the lower 

 animals, as well as on man, has at least partially cleared up the 

 mystery, although there are still unsolved problems. Rodents are the 

 most active distributors of this disease. They carry the infection from 

 house to house, from village to village, and from home ports to distant 

 lands. The first appearance of the disease in a country previously 

 free from the infection occurs in those occupied about the docks, 

 receiving grain and other commodities from infected lands. Generally 

 before a case has occurred in man the wharf rats are observed to 

 behave peculiarly and then to die in large numbers. An infected rat 

 has landed from the ship and has infected others. How does one rat 

 infect others ? The most probable answer to this question is that the 

 sick new arrivals, one or more, have died and the natives have fed 

 on the corpse and acquired the infection in this way. While a hungry 

 rat may feed on the dead of its own species, it seems to prefer other 

 food. Moreover, in a dead rat the bacilli of plague do not retain their 

 virulence for many days, the exact time varying with the temperature. 

 While a rat may be infected by feeding on a dead fellow, it is much 

 more susceptible to inoculation through the skin. The conclusion is 

 that while this method of transfer may and probably does occur, it is 

 not the sole or even the most important method, and we shall look 

 farther. 



The urine and feces of the infected rat may be deposited on grain 

 or other kinds of food and eaten by the native animal. This is a 

 possibility easily submitted to experiment. It has been tried and has 

 failed in all cases. Corn and other grains have been mixed with the 

 urine and feces of infected rats and fed to sound ones with wholly 

 negative results. 



Rats serve as hosts to several parasites, among which is the rat 

 flea (Pulex cheopis) and possibly this may serve in the transfer of the 

 bacillus from one rat to another. In the first place this flea is taken 

 from an infected rat, examined and found to contain the bacillus of 

 plague. This seems to be a step in the right direction. Such a flea 

 is crushed, a needle touched with the material is used to prick a 

 healthy rat and this animal becomes infected and dies of the plague. 



