282 BACTERIOLOGY. 



the infection is evident, from the fact that individuals 

 contracting tuberculosis from the same source are at- 

 tacked with different severity, and that there is, as a 

 rule, no great difference in degrees of virulence in the 

 tubercle bacilli obtained from different sources. As is 

 seen from the results of post-mortem examinations in 

 which the remains of old tubercular processes have been 

 found in the lungs of about one-third of all the bodies 

 examined, many cases of pulmonary phthisis must occur 

 without showing any visible evidences of disease, and 

 heal of their own accord. The possibility of favorably 

 influencing in many an existing tuberculosis by treat- 

 ment also proves that, under natural conditions, there is 

 a varying susceptibility to the disease. Clinical experi- 

 ence teaches, likewise, that poor hygienic conditions, 

 depressing surroundings (as in asylums and prisons), 

 obstinate bronchial affections, diabetes, and other ex- 

 hausting diseases increase the susceptibility to phthisis. 

 Animal experiments have shown that not only are there 

 differences of susceptibility in various animal species, 

 but also an individual susceptibility in the same species. 

 This is not so evident among guinea-pigs, which are 

 so susceptible that they succumb to an inoculation of 

 the minutest dose of virulent bacilli; but rabbits are 

 not always killed by subcutaneous inoculations, though 

 some individuals die from very small doses. Dogs, 

 rats, and other more resistant animals show this still 

 more plainly. Man cannot be placed on the same plane 

 of susceptibility to tuberculosis with guinea-pigs, for 

 with him the disease often remains local or is entirely 

 cured. The doctrine of individual susceptibility, there- 

 fore, is seen to be founded on fact, although the reasons 

 for it are only partially understood. 



