04 THE RELATION OF BACTERIA TO DISEASE 



number of bacteria overcome the primary defenses 

 and yield when the reserve powers have been brought 

 into play. Again, a low grade of virulence may be 

 possessed by the invaders, and although many enter, 

 the specific disease process is halted by the economy. 

 Moreover, some individuals seem to be poor hosts 

 for certain bacteria, while others are received readily. 

 The general resistance of the body to disease is spoken 

 of as immunity. Immunity as the term is usually 

 used means that an individual is not susceptible to a 

 disease, but not necessarily that he would not be 

 infected under any severe circumstances. 



Types of Immunity. Immunity is classified as (1) 

 natural, or racial or species immunity, and (2) acquired 

 immunity, which latter has been further divided into 

 active and passive. 



Natural immunity is the condition wherein a certain 

 disease does not occur in the type of animal under 

 consideration, as for example, the dog does not take 

 typhoid fever even when fed a pure culture of the 

 specific germs. There is also a relative natural im- 

 munity. Cats present great resistance to infection 

 with anthrax. 



Racial immunity is shown by great resistance of the 

 negro to yellow fever. 



There is also individual immunity as shown by the 

 passing of a person through a virulent epidemic without 

 the slightest sign of illness. 



Acquired immunity is that resistance which a 

 person obtains by passing through an attack of disease. 

 That a second attack of measles or scarlatina seldom 

 occurs is well known. This is seen also in typhoid 



