70 THE RELATION OF BACTERIA TO DISEASE 



foods may be little or not altered by these poisonous 

 substances in them. They are in small quantity in 

 the food, but are easily and quickly absorbed. It is 

 possible that for a short time after ingestion of the 

 meat the formation of these ptomains may continue. 

 The ptomains are toxins, but they are formed by alter- 

 ing the chemical composition of the meat rather than 

 by any peculiar products of the bacteria- or poisons 

 within their bodies. The condition is not transmissible. 



IMMUNITY. 



The resistance offered to the entrance of micro- 

 organisms 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 con- 

 sidered. It is a well-known fact that illness does not 

 occur every time pathogenic bacteria gain a foot- 

 hold 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. 



