The Cardinal Conditions of Infection 91 



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 in- 

 fectious 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 open- 

 ing of a privy, while in others the excreta of the animals were allowed 

 to accumulate, suffered from a pronounced diminution of the re- 

 sisting 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 susceptibility to 

 disease, and experimental evidence of its correctness is not wanting. 

 Charrin and Rogerf found that white rats, which naturally resist 

 infection with anthrax, succumbed to the infection if compelled to 

 turn a revolving wheel until exhausted before inoculation. 



Exposure to cold seriously diminishes the resisting power of the 

 warm-blooded animals. It is an everyday experience that chilling 

 the body predisposes to "cold" and may be the starting-point of 

 pneumonia. Pasteur found that fowls, which resist anthrax under 

 normal conditions, succumbed to infection if kept, for some time, in 

 a cold bath before inoculation. 



The reverse seems to be true of the cold-blooded animals, for 

 Gibier§ found that frogs, naturally resistant to the anthrax bacillus, 

 would succumb to infection if kept at 37'^C. after inoculation. 



Diet produces some variation in the resisting powers. The 

 tendency of scorbutics to suffer from infectious disorders of the 

 mouth, the frequency with which epidemics of infectious disease fol- 

 low famines, and the enterocolitis of marasmatic infants, illustrate 

 the effects of insufficient food in predisposing to disease. We also 

 find that the infectious diseases of carnivorous animals are not the 

 same as those of herbivorous animals, and that the former are 

 exempt from many disorders to which the latter quickly succumb. 

 Hankin was able to show experimentally that meat-fed rats resisted 

 anthrax infection far better than rats fed upon bread. 



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



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



X "Compte rendu Soc. de Biol, de Paris," Jan. 24, 1890. 



§ "Compte rendu Acad, des Sciences de Paris," 1882, t. xcix, p. 1605. 



