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 evideut 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. 
