204 GENERAL BIOLOGY Ol? MICRO-ORGANISMS 



some instances the adaption is very narrow and the parasite 

 may be able to exist naturally only in the one-host species, as for 

 example Spirochceta pallida. Individual resistance of different 

 hosts of the same species is variable. Age is one important fac- 

 tor: there are the children's diseases, measles, chickenpox; the 

 diseases of active adult life, pulmonary tuberculosis, typhoid 

 fever ; and the diseases of the aged, pneumonia, carcinoma. Hun- 

 ger and thirst have been shown experimentally to reduce the 

 resistance to infection: pigeons, which are normally immune to 

 anthrax become susceptible when starved. The effect of fatigue 

 is well known: a white-rat, normally immune to anthrax, suc- 

 cumbs to it after prolonged work in the treadmill. Abnormal 

 chilling of hens removes their immunity to anthrax and abnormal 

 heating of frogs affect them in a similar way. .Chemical poisoning 

 also increases susceptibility to infection, and cachexia and mal- 

 nutrition are well-known predisposing factors to such infections 

 as tuberculosis. Traumatism is very important, not only for its 

 general effect upon the resistance of the host, but especiallyin the 

 reduction of local resistance by destruction or injury of tissue 

 (wounds). There are certain locations where resistance to in- 

 fection is naturally lower, such as the ends of growing bqnes and 

 the interior of the parturient uterus. 



Niimber of Invaders.' — The quantity of infectious material 

 introduced is of importance in determining whether infection 

 will or will not occur. Very few species of microbes are capable 

 of causing disease when only a single individual organism is in- 

 troduced into the body. A large number of microbes entering 

 at the same time seems to overburden the defensive powers of 

 the body so that some of the parasites succeed in establishing 

 themselves and multiplying. 



Modes of Introduction. — There are various avenues; by which 

 micro-organisms may enter the body to produce disease. In- 

 fection of the ovum in the ovary with spirochetes and protozoa 

 is known to occur in some insects, and Rettger has shown that 

 this phenomenon occurs in the hen infected with Bacterium pul- 



