PARASITISM AND PATHOGENESIS 1 97 



some instances the adaptation is very narrow and the parasite 

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

 example Spirochaeta pallida. Individual resistance of different 

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

 factor: there are the children's diseases, measles, chicken- 

 pox; the disease of active adult life, pulmonary tuberculosis, 

 typhoid fever; and the diseases of the aged, pneumonia, carci- 

 noma. Hunger 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, succumbs to it after prolonged work in the treadmill. 

 Abnormal chilling of hens removes their immunity to anthrax 

 and abnormal heating of frogs affects them in a similar way. 

 Chemical poisoning also increases susceptibility to infection, and 

 cachexia and malnutrition are well-known predisposing factors 

 to such infections as tuberculosis. Traumatism is very impor- 

 tant, not only for its general effect upon the resistance of the host, 

 but especially in the reduction of local resistance by destruction 

 or injury of tissue (wounds). There are certain locations where 

 resistance to infection is naturally lower, such as the ends of 

 growing bones and the interior of the parturient uterus. 



Number 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- 



