BACTERIA AND DISEASE 405 



that they cause. The answer is found in the remarkable protective 

 devices of the human body. To cause disease, a bacterial organ- 

 ism must first gain entrance into a body, and in the case of man, 

 such a successful entrance is often very difficult. Bacteria of the 

 air are filtered out during breathing by hairs in the nostrils, and 

 by the mucous linings of the nose and throat so that they rarely 

 reach the delicate network of air sacs in the lungs. Bacteria in 

 foods have to withstand the acid of the gastric juices in the stom- 

 ach, later the alkali of the bile secretions poured in from the liver 

 and finally the ravages of other bacteria which are normal 

 residents of the human intestines. The skin affords ample pro- 

 tection to the outside of the body, being dry and impenetrable 

 to bacteria except when broken. 



In spite of all these protective devices, some pathogenic 

 bacteria do gain entrance into the tissues of the human body, 

 where they meet a secondary line of defense which is likewise 

 very effective. The blood contains substances which weaken most 

 bacteria, making them easy prey for the white corpuscles which 

 then destroy them. The strength of these defenses varies from 

 person to person, and from time to time in the same person 

 depending upon the physical condition. This internal resistance 

 to attack by disease causing bacteria is known as immunity. If 

 an individual can resist attack by certain disease bacteria, he is 

 said to be immune to the disease; if he succumbs easily, he is 

 said to be susceptible to the disease. Disease itself is not inherited, 

 but immunity or susceptibility to a particular disease can be 

 inherited. When a person is born with an immunity he is said 

 to possess a natural immunity. Thus the white race has a certain 

 amount of natural immunity to measles, and it is relatively mild 

 in its effects on individuals of this race, but when brought to the 

 American Indians and natives of Greenland and Iceland it 

 proved a very deadly disease among them. The same is true of 

 the smallpox brought to the American Indians by European 

 settlers. 



The immunity which results from having had a particular 

 type of disease is known as natural acquired immunity. Such 

 an immunity is brought about by the development in the human 

 body of certain substances, which react against either the 



