THE CONQUEST OF DISEASE 



lives in the blood and lymph vessels of the rat 

 and in the intestinal canal of the flea which 

 bites the rat. This knowledge has made fight- 

 ing the disease possible. It means the extermi- 

 nation at least of the rat. When the rat con- 

 tracts plague he sickens and dies. Fleas re- 

 quire living blood. When their hosts have 

 perished they seek men or other living animals, 

 and so carry the infection. 



An interesting experiment is made with 

 guinea pigs. If the animals are put in little 

 cages in a plague-infected room, the cages 

 being two feet from the floor, the guinea pigs 

 do not contract the disease. The flea can not 

 jump two feet. But if the cages are hung two 

 inches from the floor the animals develop the 

 disease and die. In India when sick rats are 

 observed leaving the cellars, the people either 

 desert the house or seek the roof, for as soon 

 as the last rat in the house is dead they are in 

 danger of being sought by the starving fleas. 

 In the fall of 1908 two little boys in California 

 found a recently dead rat. They put it in a 

 box and buried it. Within a week the family 

 of seven persons fell ill. The rat was dug up 

 and found to have died of plague. Father, 

 mother, grandmother, and four children died. 



72 ' 



