492 MAN CONTROLS HIS ENVIRONMENT FOR HEALTH 



of the Middle Ages, a disease estimated to have killed 25,000,000 

 people during the fourteenth century. Fleas bite the infected 

 rat and then transmit the disease to man. In 1900 the plague 

 gained entrance on our western coast. It killed more than 100 

 persons during the next four years, and small outbreaks have 

 occasionally occurred ever since. The ground squirrels of Cali- 

 fornia became infected with the plague, doubtless from the rats 

 which lived in their burrows, so that now the danger of other out- 

 breaks of the plague will be present until all the ground squirrels 

 are exterminated. Over a million rats were killed in fighting the 

 last outbreak of bubonic plague in California and efforts are being 

 made in all large cities to eradicate this pest. 



Practical Exercise 22. Look up Farmers Bulletin 896 and report on the 

 best way to exterminate rats. 



Project. Make a survey of your neighborhood to determine where 

 rats breed. 



Self-Testing Exercise 



The house fly may carry (1) and (2) bacteria 



on its feet. It breeds (3) in (4) (5) during 



the warm season. Fleas are carriers of (6) (7), 



which they get from (8) (9) and (10) 



(11). This disease can be eradicated by exterminating all 



(12) and (13). Body lice transmit (14) 



(15). 



PROBLEM X. WHAT DISEASES ARE CAUSED BY WORMS 

 AND HOW MAY WE FIGHT THEM? 



Other parasitic animals cause disease. Other animals besides 

 those mentioned have been found to cause illness. Chief among 

 these are certain roundworms and flatworms, which live as parasites 

 not only in man but in many animals and plants. The parasite 

 frequently becomes fastened to its host during adult life and is 

 reduced to a mere bag through which the fluid food prepared by its 

 host is absorbed. Sometimes a complicated life history results 

 from parasitic habits. Such is the life history of the tapeworm 

 and of the liver fluke, a flatworm which kills sheep. 



