HEALTH AND DISEASE 45 



Similarly an organism possessing the same degree of virulence 

 may cause little or no effect in one person whose immunity or 

 power of resistance is relatively high ; it may produce a slight 

 attack of disease in another whose immunity is less marked ; 

 and a severe and perhaps a rapidly fatal illness in a third whose 

 vitality is lowered by various causes such as those enumerated 

 above, or who does not possess a certain degree of natural im- 

 munity against the disease. In these instances we have been 

 discussing individual immunity, but there is also such a thing 

 as family and race immunity, and again some animals are 

 liable to, whilst others are relatively, or even almost absolutely, 

 immune to certain diseases. Thus, foul-feeding animals such 

 as fish, vultures, ravens, and carnivorous animals in general 

 are relatively immune to bacterial infection. On the other hand 

 cattle, and some rodents, especially guinea-pigs, are extremely 

 susceptible to tuberculosis and anthrax, rabbits and mice to 

 infection with the pneumococcus, and so on. 



In the same way certain races of mankind are more immune 

 or more liable to certain diseases than others. One well-known 

 example of this is, when civilised races have invaded and 

 colonised the countries of other and simpler nations, the 

 " diseases of civilisation " have in many cases decimated and in 

 some instances almost annihilated the latter. Thus tuberculosis 

 is specially virulent amongst negroes ; and a somewhat similar 

 phenomenon was seen during and after the South African War, 

 when measles in a very virulent form broke out among the 

 Dutch population, particularly in the concentration camps, and 

 carried off many victims who had not in infancy contracted the 

 disease and so become immune, as is generally the case amongst 

 ourselves. Again, diseases to which the native population may 

 be partially or almost entirely immune, probably partly by 

 heredity and partly through having overcome the disease early 

 in life, may attack the invading races with special virulence. 

 Malarial and yellow fevers may be instanced as examples of 

 this. 



Many diseases are common to men and animals ; e.g. tuber- 

 culosis, plague, and the like. Some infections such as glanders 

 and anthrax are found amongst animals, but are occasionally 

 communicated to man ; and again man is practically immune 

 to certain animal diseases, such as hog- and chicken-cholera ; 

 whilst such human diseases as measles, whooping-cough, yellow- 

 fever, syphilis, gonorrhoea, etc., do not naturally attack the 

 lower animals. 



Infective disease is, then, merely one instance of the struggle 

 in nature, wherein one living being preys upon another for 

 food ; and in this case it is the attack of the relatively small 



