622 THE POPULAR SCIENCE MONTHLY. 



failed to clear up the etiology of the disease. In the last-men- 

 tioned disease, there are excellent a priori reasons for believing 

 that a living micro-organism of some kind is the essential etiologi- 

 cal factor ; but this hypothetical germ has eluded all researches. 

 Possibly it belongs to an entirely different class of micro-organ- 

 isms, as is the case with the blood parasite which is now recog- 

 nized as the cause of the malarial fevers. 



Having thus briefly reviewed the progress of our knowledge 

 relating to the etiology of infectious diseases, I desire to call your 

 attention to the question of acquired immunity from these diseases. 



No questions in general biology are more interesting, or more 

 important from a practical point of view, than those which 

 relate to the susceptibility of certain animals to the pathogenic 

 action of certain species of bacteria, and the immunity, natural 

 or acquired, from such pathogenic action which is possessed by 

 other animals. It has long been known that certain infectious 

 diseases, now demonstrated to be of bacterial origin, prevail only 

 or principally among animals of a single species. Thus, typhoid 

 fever, cholera, and relapsing fever are diseases of man, and the 

 lower animals do not suffer from them when they are prevailing 

 as an epidemic. On the other hand, man has a natural immunity 

 from many of the infectious diseases of the lower animals, and 

 diseases of this class which prevail among animals are fre- 

 quently limited to a single species. Again, several species, in- 

 cluding man, may be susceptible to a disease, while other animals 

 have a natural immunity from it. Thus, tuberculosis is common 

 to man, to cattle, to apes, and to the small herbivorous animals, 

 while the carnivora are, as a rule, immune ; anthrax may be com- 

 municated by inoculation to man, to cattle, to sheep, to guinea- 

 pigs, rabbits, and mice, but the rat, the dog, carnivorous animals, 

 and birds are generally immune ; glanders, which is essentially a 

 disease of the equine genus, may be communicated to man, to the 

 guinea-pig, and to field-mice, while house-mice, rabbits, cattle, 

 and swine are to a great extent immune. 



In addition to this general race immunity or susceptibility, we 

 have individual differences in susceptibility or resistance to the 

 action of pathogenic bacteria, which may be either natural or 

 acquired. As a rule, young animals are more susceptible than 

 older ones. Thus in man the young are especially susceptible to 

 scarlet fever, whooping-cough, and other "children's diseases," 

 and after forty years of age the susceptibility to tubercular in- 

 fection is very much diminished. Among the lower animals it is 

 a matter of common laboratory experience that the very young of 

 a susceptible species may be infected when inoculated with an 

 "attenuated culture" which older animals of the same species 

 are able to resist. 



