12 BIOLOGICAL LECTURES, 



By " natural immunity" we mean the insusceptibility which 

 certain species or certain individuals have against infection by 

 certain pathogenic bacteria. " Acquired immunity " is the 

 resisting power which results, in originally susceptible animals, 

 from a non-fatal attack of an infectious disease, or from pro- 

 tective inoculations with " attenuated" or filtered cultures of 

 pathogenic bacteria. 



As is well known, certain infectious diseases are peculiar to 

 man, certain others to one or more species of lower animals, 

 and others still may be transmitted from man to certain lower 

 animals, or vice versa. Thus typhoid fever, yellow fever, 

 cholera, measles, etc., are infectious diseases of man, and 

 during their epidemic prevalence the lower animals show no 

 evidence of susceptibility to infection. On the other hand, 

 Texas fever and infectious pleuropneumonia, which are very 

 fatal to cattle, hog cholera and swine plague, chicken cholera, 

 etc., are never communicated to those who Care for the infected 

 animals ; in other words, man has a natural immunity against 

 these diseases. Again, certain infectious diseases are common 

 to man and to certain species of the lower animals. Thus, 

 tuberculosis may be transmitted to monkeys, to cattle, rabbits, 

 guinea pigs, and fowls ; carnivorous animals in confinement, 

 also, sometimes succumb to tubercular infection. Anthrax 

 is fatal to cattle, sheep, and small herbivorous animals, and 

 may be transmitted to man by inoculation, or by the introduc- 

 tion of dry spores into the respiratory passages " wool- 

 sorter's disease." Diphtheria may be transmitted to cats and 

 fowls, and cultures of the diphtheria bacillus are extremely 

 pathogenic for the guinea pig. Glanders, which is a disease of 

 the equine genus, may be transmitted to man, to the guinea pig, 

 and to the field mouse ; but house mice have a natural immunity 

 against infection by this bacillus. On the other hand, field 

 mice and guinea pigs are not susceptible to infection by the 

 Bacillus erysipelatus suis, which is very fatal to house mice, 

 white mice, rabbits, swine, sparrows, and pigeons. There are 

 also differences in susceptibility to various infectious diseases 

 among different races of the same species. Thus the Algerian 

 sheep has an immunity against anthrax ; Texas cattle, as a 



