4IO ELEMENTARY BIOLOGY 



443. Other disease-bearing insects. Fleas have been implicated in 

 a very serious combination injurious to man. The bubonic plague, 

 which has in past times been the most dreaded of diseases, especially 

 in Asia, was found (in 1894) to be caused by a specific bacillus. But 

 the mode of infection was not known until quite recently. The Chinese 

 had observed, centuries ago, that there was some connection between 

 the dying of rats in large numbers and the appearance of the plague. 

 Modern scientists set to work to find out whether the rat plague was 

 in any way related to the human plague, and they found that the same 

 bacillus is the cause of both. Then it was found out that the plague 

 spreads from rat to rat not by contact of the animals but through fleas 

 that suck the blood of sick rats and later bite others, thus transferring 

 the infection. Further observation showed that the plague is primarily 

 a disease of rats, and gets into human beings when the fleas abandon 

 dead rats and infect men and women. The plague has spread from 

 the Orient, and cases have appeared at several ports in the United 

 States within a few years. The methods developed for dealing with 

 this danger are directed not toward killing the bacteria but toward 

 killing the rats and fleas. A ship coming from an affected port is 

 thoroughly fumigated to kill the fleas and rats, special devices are 

 attached to ropes and chains to keep rats from getting ashore (or 

 aboard, for that matter), and a search is made for hiding places in 

 which rats may be concealed. In California it was found that the 

 ground squirrels have become infected with the plague bacillus, and 

 systematic patrols are established to catch rats and ground squirrels, 

 which are regularly examined for possible infection. 



Since the L^nited States joined the Great War, many important 

 medical problems have been solved by our investigators. One of 

 these had to do with the transmission of trench fever, a disease that 

 caused a great deal of suffering and incapacity, although it was 

 seldom fatal. Volunteers from the ambulance and field-hospital units 

 allowed the blood of patients to be injected into their veins. The 

 development of the disease in the men so treated showed that the 

 sickness is due to a germ (too small to be seen with the microscopes) 

 present in the blood. Other volunteers allowed themselves to be 

 bitten by lice taken from the bodies of sick men, and many of these 

 developed the disease, while others, bitten by lice from healthy men. 

 remained unaffected while living under exactly the same conditions. 

 This showed that the germs are carried over by means of the louse. 



