108 APPLIED BACTERIOLOGY 



pathogenic organism, the Bacillus nnallei, for example, 

 while many animals exhibit more or less susceptibility, 

 some are incapable of being affected at all ; such animals 

 are said to possess a ' natural immunity ' to glanders. 



Anthrax, again, is very fatal to ordinary sheep and many 

 other animals, while Algerian sheep enjoy a complete 

 immunity. 



The lower animals possess a complete immunity to 

 several important diseases to which man is susceptible, 

 namely, leprosy, syphilis, gonorrhoea, cholera, etc. 



The carnivora are remarkable in enjoying a considerable 

 degree of immunity against the organisms of septicsemia, 

 to which the herbivora are far more susceptible. 



Confining our attention more particularly to the diseases 

 affecting man, we find that in the case of several, notably 

 small-pox, measles, mumps, whooping-cough, and scarlet 

 fever, it is comparatively rare for the same person to be 

 attacked twice by the same disease. That is to say, one 

 attack is ' protective,' and in the above-mentioned diseases 

 the ' protection ' usually lasts a lifetime. 



On the other hand, an attack from certain other diseases 

 does not confer this protection, but rather predisposes 

 the patient to the second attack of the same disease. This 

 is true of influenza, diphtheria, pneumonia, and malaria. 



To return to the group of diseases before mentioned, in 

 which protection is conferred by an attack, this protection 

 extends only to that particular disease, and does not in any 

 way protect against other diseases; while in the second 

 group, in which no protection is afforded, we find that 

 there may be not only predisposition to a second attack 

 of the same disease, but even a distinct predisposition to 

 attack by other diseases — thus, diphtheria and scarlet fever 

 mutually predispose to one another. 



Hypotheses of Immunity. — Before examining in detail the 



