IMMUNITY. 103 



is reduced by disease and the amount of infection is 

 great. The inability of a micro-organism to grow 

 in the body of an animal does not usually indicate, 

 however, an insusceptibility to its poison; thus, for in- 

 stance, rabbits are less susceptible than dogs to the effects 

 of the poison elaborated by the pneumococci, but these 

 bacteria develop much better in the former than in 

 the latter. Differences in susceptibility are sometimes 

 very marked among different varieties of the same race 

 of animals, as, for instance, between different kinds of 

 rats and pigeons to anthrax. In animals, as a whole, 

 it is noticed experimentally that the young of all species 

 are less resistant to infection than the older and larger 

 ones. 



The difficulty experienced by the large majority of 

 bacteria in developing in the tissues of the healthy 

 body can be to a great extent removed by any cause 

 which lowers the general or local vitality of the tissues. 

 Among the causes which bring about such lessened re- 

 sistance of the body are hunger and starvation, bad 

 hygienic surroundings, exhaustion from overexertion, 

 exposure to cold, the deleterious effects of poisons, bac- 

 terial or other, acute and chronic diseases, vicious 

 habits, drunkenness, etc. Purely local injuries, such 

 as wounds, contusions, etc., also give sometimes a point 

 of entrance for infection, or at least a point of less re- 

 sistance, where the bacteria may develop and produce 

 local inflammation. This is noted in infection by the 

 tubercle and typhoid bacilli, pyogenic cocci, etc. 

 Local affections, such as endocarditis, may also afford 

 a weak spot for the bacteria to seize upon. The pres- 

 ence of foreign bodies in the tissues in like manner 

 predisposes them to bacterial invasion. Interference 



