IMMUNITY 65 



by any peculiar products of the bacteria, and they are not 

 specific for any particular variety of microorganism. Ptomain 

 poisoning is not transmissible. 



IMMUNITY. 



The resistance offered to the entrance of microorganisms 

 into the body has already been referred to, and now the 

 method by which our physiology gets rid of the effects of 

 these noxious agents must be considered. It is a well-known 

 fact that illness does not occur every time pathogenic bacteria 

 gain a foothold on or within the body. Sometimes a small 

 number of bacteria overcome the primary defences 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 very 

 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 immunity. 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. 

 5 



