286 THE RELATIONS OF ANIMALS TO DISEASE 



unswept corner may hold the eggs of the flea. The young breed in 

 cracks and crevices, feeding upon organic matter there. Eventu- 

 ally they come to live as adults on their warm-blooded hosts : cats, 

 dogs, and man. Evidently destruction of the breeding places, 

 careful washing of all infected areas, the use of a creosote prepara- 

 tion in crevices where the larvae may be hidden, are the most effec- 

 tive methods of extermination. Pets which might harbor fleas 

 should be washed frequently with a weak (two to three per cent) 

 solution of creolin. It is thought probable that bedbugs spread 

 typhus and relapsing fevers. They prefer human blood to other 

 food and live in bedrooms because this food can be obtained 

 there. They are extremely difficult to exterminate because their 

 flat bodies allow them to hide in cracks out of sight. Wooden 

 beds are thus better protection for them than iron or brass beds. 

 Boiling water poured over the cracks where they breed, or a mix- 

 ture of four parts of strong corrosive sublimate with four parts of 

 alcohol and one part of spirits of turpentine is an effective remedy. 



Bdcillus pesfis 



Diagram showing how bubonic plague is carried to man. 



Animals Other than Insects may be Disease Carriers. — The 



common brown rat is an example of a mammal, harmful to civilized 

 man, which has followed in his footsteps all over the world. Start- 

 ing from China, it spread to eastern Europe, thence to western 

 Europe, and in 1775 it had arrived in this country. In seventy- 

 five years it reached the Pacific coast and it is now fairly common 

 all over the United States, being one of the most prolific of all 

 mammals. Rats carry bubonic plague, the '^ Black Death '^ 

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

 people during the fourteenth century. Plague is primarily a dis- 

 ease of rats. Fleas bite the rat and then transmit the disease to 



