8o Infection 



of overcoming the invasiveness of certain parasitic micro- 

 organisms, and are so naturally susceptible; others are 

 normally able to resist their inroads, but because of various 

 temporary or permanent physiological changes in their con- 

 stitution, lose such 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 disease. These 

 changes are often so subtile that they escape detection, 

 though at times they can be partly understood. 



Depressing hygienic conditions of all kinds are shown by 

 statistics to predispose to infectious diseases of all kinds. 

 So many factors are combined in this broad statement, 

 however, that it is difficult to subject them to a satisfactory 

 analysis. Doubtless the expression stands for the sum of 

 all the conditions that can be separately considered. 



(a] The inhalation of noxious vapors. It has long been 

 supposed that sewer gas was responsible for the occurrence 

 of many infectious diseases, and when the nature of such 

 diseases was made clear by a more complete knowledge of 

 their bacterial causes, the fact still seemed to remain that 

 defective sewage was in some way connected with the inci- 

 dence of disease. It is difficult to prove its truth experi- 

 mentally and the relation may be traditional and imaginary, 

 though it is still commonly supposed that the vapors of sewage 

 produce some depression of the vital powers. Men who work 

 in sewers and plumbers who breathe much sewer gas are not 

 apparently affected. 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 

 dimunition of the resisting powers. This would seem to 

 be inconsistent with the habits of rats, many of which live 

 in sewers. Abbottf 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 de- 

 composition inhaled by the animals played no part in pro- 

 ducing disease, or in inducing susceptibility to it. 



(b) Fatigue is a well-recognized clinical cause of sus- 

 ceptibility to disease, and experimental evidence of its 



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

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



