The Cardinal Conditions of Infection 101 



mals lived longest, the liver seeming to be far less destruc- 

 tive to streptococci than the lungs. 



The Susceptibility of the Host. Susceptibility is lia- 

 bility to infection. It is a conditon in which the host is 

 unable to defend itself against invading micro-organisms. 

 Unusual or unnatural susceptibility is also spoken of as 

 predisposition or dyscrasia. 



Many animals and plants are naturally without any means 

 of overcoming the invasiveness of certain parasitic micro- 

 organisms, and are, therefore, naturally susceptible; others 

 naturally resist their inroads, but through various temporary 

 or permanent physiologic changes may lose the defensive 

 power. 



In general, it is true that any condition that depresses or 

 diminishes the general physiological activity of an animal 

 diminishes its ability to defend itself against the pathogenic 

 action of bacteria, and so predisposes to infection. These 

 changes are often so subtile that they escape detection, 

 though at times they can be partly understood. 



The inhalation of noxious -vapors. It has long been 

 supposed that sewer gas was responsible for the occurrence 

 of certain infectious diseases, and when the nature of these 

 diseases was made clear by a knowledge of their bacterial 

 causes, the old belief still remained and many sanitarians 

 continued to believe that defective sewage is in some way 

 connected with their occurrence. It is difficult to prove 

 or disprove the matter experimentally. Men who work 

 in sewers and plumbers who breathe much sewer gas are 

 not apparently affected by it. Alessi* found that rats, 

 rabbits, and guinea-pigs kept in cages some of which were 

 placed over the opening of a privy, while in others the 

 excreta of the animals were allowed to accumulate, suffered 

 from a pronounced diminution of the resisting powers. 

 This would seem to be inconsistent with the habits of rats, 

 many of which live in sewers. Abbott f caused rabbits to 

 breathe air forced through sewage and putrid meat infusions 

 for one hundred and twenty-nine days, and found that the 

 products of decomposition inhaled by the animals played no 

 part in producing disease, or in inducing susceptibility to it. 



Fatigue is a well-recognized clinical cause of suscep- 

 tibility to disease, and experimental evidence of its 



*"Centralbl. f. Bakt.," etc., 1894, xv, p. 228. 

 t" Trans. Assoc. Amer. Phys.," 1895. 



