CONTAGIOUS DISEASES 379 



person has been bitten by a mad dog, some time elapses 

 before the symptoms of the disease appear. During this 

 period the person is inoculated first with a weakened virus 

 of hydrophobia which causes the body to form an antibody, 

 then with increasingly stronger doses so that the body 

 gradually becomes accustomed to these attacks, until by 

 the time the effects from the dog bite would have been felt 

 the body is able to overcome them entirely. 



Typhoid fever. There are two diseases of common 

 occurrence which are the cause of so many deaths that 

 special mention should be made of them. They are typhoid 

 fever and tuberculosis. They are due largely to people's 

 carelessness and ignorance and hence are preventable dis- 

 eases. Typhoid fever germs are often spread through drink- 

 ing water. These bacteria live in the intestines of the patient 

 and pass out from the intestines in the excreta, both the 

 feces and the urine. They may pass directly with the 

 sewage into some stream, or they may be deposited on the 

 soil, from which they may sink down into the underground 

 water and eventually find their way into wells. It fre- 

 quently happens in the country that the well is located 

 very near the place which receives the sewage, and the 

 bacteria may pass into the well to be taken into the bodies 

 of people who drink the water. 



In cities which have a system of sewers, it sometimes 

 happens that the sewers empty into a stream from which a 

 city lower down takes its drinking water. When typhoid fever 

 breaks out in the first city, the bacteria are carried to the 

 second city, where another outbreak occurs. It is seen thus 

 that the prevention of typhoid fever depends largely on two 

 things, the proper disposal of sewage, and the purification 

 of drinking water. Some facts about the purification of 

 water were given in a previous chapter. 



Recovered patients a source of infection. Another con- 

 sideration which emphasizes the need of continual care in 



