IMMUNITY. 1 3 



rule, do not succumb to Texas fever, which is very fatal to 

 northern cattle ; yellow fever is much less fatal to the negro 

 than to the white race ; the negro also resists malarial infection 

 better than the white. In general, carnivorous animals have 

 but slight susceptibility to the various forms of infectious 

 septicaemia which are fatal to the herbivora. The introduction 

 beneath the skin of a mouse or a rabbit of a little putrefying 

 flesh infusion frequently gives rise to a fatal septicaemia, due 

 to the presence of one or more species of bacteria pathogenic 

 for these animals, but harmless for the carnivora which prey 

 upon them. Were this otherwise, the carnivora would be 

 destroyed by wounds inflicted in their fights over animals 

 which had died of infectious diseases or which were in a state 

 of putrefaction. It appears probable that the immunity of the 

 carnivora to the infectious septicaemias referred to has been 

 acquired in process of time by natural selection — survival of 

 the fittest. 



Besides the race immunity referred to, we have differences 

 in the degree of susceptibility to various infectious diseases in 

 individuals of the same race. Some individuals or families 

 have manifestly a special susceptibility to tubercular infection ; 

 others are especially subject to contract smallpox, as shown by 

 the occurrence of two or more attacks in the same individual. 

 Susceptibility also depends to some extent upon age. The 

 young are especially susceptible to scarlet fever, whooping 

 cough, and diphtheria. In the lower animals, we find that an 

 "attenuated virus" which will not kill an adult of a susceptible 

 species may prove fatal to a very young animal of the same 

 species. 



Immunity, whether natural or acquired, in many cases has 

 only a relative value, and may be overcome by an excessive 

 dose or by unusual virulence of the infectious material. This 

 is true, for example, of the natural immunity of the Algerian 

 race of sheep as regards anthrax, of the acquired immunity 

 resulting from vaccination against smallpox, etc. But it does 

 not apply in the case of animals which have a complete natural 

 immunity for infectious diseases peculiar to other species. 

 Thus man never contracts swine plague or infectious pleuro- 



