8 NATURAL IMMUNITY 



the lower animals. In others different types of the infecting 

 organism occur, and a given species is susceptible to one, immune 

 to others ; for example, there are three, and perhaps more, varieties 

 of tubercle bacillus, which resemble one another in many points, 

 and which attack respectively man, cattle, and birds, and each 

 animal species is more or less immune to bacilli from animals far 

 removed in the scale. 



In general terms, the immunity or susceptibility of different 

 animals depends to some extent on their zoological affinities. 

 Thus man is pre-eminently susceptible to the Spirochata pallida, 

 the anthropoid apes less so, but still not immune, and the lower 

 animals entirely refractory. Rinderpest affects cattle, sheep, 

 goats, and other ruminants, and South African horse-sickness 

 horses, asses, and mules. But to this rule there are numerous 

 exceptions : thus, almost all warm-blooded animals are susceptible 

 to anthrax, but the Algerian sheep and white rat are relatively 

 immune, the wild rat being susceptible. And of the domestic 

 animals we find cattle to be highly susceptible to tubercle, whereas 

 goats, though closely allied zoologically, are almost immune. 



Natural immunity does not exist to an equal degree in all 

 individuals of a species. This is well seen in man during an 

 epidemic, where, of a certain number of persons who are exposed 

 to an infection (and, as far as we know, receive the same dose of 

 the materies morbi), some escape the disease altogether, some have 

 a slight, and others a severe, attack, whilst yet others die rapidly. 

 Sex has some influence here, but it is usually difficult to trace, 

 since the males and females of a community are in most cases 

 exposed to an infection in varying degree. 



Age is of more importance, and, in quite general terms, we may 

 say that the younger the infant the less its immunity. Certain 

 diseases, such as measles, scarlet fever, and whooping-cough, are 

 rarely seen except in infants, and this is not altogether due to 

 acquired immunity preventing a second attack in later life. 

 Epidemic diarrhoea due to bacilli of the dysentery group is 

 rarely seen in this country, at least except in the early years of 

 life, and the same is true of cerebro-spinal meningitis and some 

 other diseases. It is also interesting to notice that the variation 

 in immunity may take a qualitative rather than a quantitative 

 form. The best example is in the case of the pneumococcus. 

 This organism is the chief cause of suppurative processes of 

 whatever region in infants, whereas in adults it is (except in 



