DURATION OF INFECTION 229 



moreover, how improved sanitation, better housing, 

 prevention of overcrowding, sewering, and destruction 

 of filth will diminish the natural hunting grounds of the 

 rat, and therefore of its flea, and will in consequence 

 lessen the chance of spreading the disease ; and this is 

 why plague has died out in places where hygiene is 

 good and survives where there still exist overcrowding, 

 squalor, and dirt. Recent investigations point to the 

 very significant fact that when once a flea is infected 

 it remains infected for a very long period, in this 

 respect resembling the other insect pests. These facts 

 explain the origin of sporadic cases of disease. 



TICK FEVER 



This is the second of the diseases communicated by 

 vermin. The name is of comparatively recent intro- 

 duction, but the disease in some form has been known 

 for many years. The relationship of ticks to the 

 propagation of disease has been worked out most 

 carefully in animals. Investigations by Drs. Smith, 

 Kilborne, and Stiles in the United States showed 

 that Texas cattle fever or red-water was sj)read 

 from animal to animal by the bites of the ticks with 

 which thev were often covered. A tick bites an 

 infected animal, and by so doing infects itself. In 

 the case of the female tick the infection is passed 

 on to the off*spring, which, being infected, are capable 

 of transmitting the virus to healthy beasts, and so the 

 disease is spread. 



As so often happens, that which occurs in the 



