Virulence 371 



tuberculosis the process of destruction is greatly accelerated 

 by inspired saprophytic bacteria that live in the necrotic 

 tissue. The patient also suffers from secondary infections, 

 especially by the streptococcus and pneumococcus. 



Most cases of tuberculosis steadily advance but a certain 

 number may recover. 



About the center of a typical tubercle is a zone of reaction 

 in which the reparative tendency of the tissue is usually 

 outweighed by the invasive power of the bacilli. 



But if the vital condition of the individual become so 

 changed that the invasive activity of the bacilli is checked 

 or their death brought about, the tubercle begins to cica- 

 trize, and becomes surrounded by a zone of newly formed 

 contracting fibrillar tissue, by which it is circumscribed 

 and isolated. This constitutes recovery from tuberculosis. 

 Sometimes the process of repair is accomplished without 

 the destruction of the bacilli, which are incarcerated and 

 retained. Such a condition is called latent tuberculosis, and 

 may at a future time be the starting-point of a new 

 infection. 



Virulence. The virulence of tubercle bacilli varies 

 considerably according to the sources from which they are 

 obtained. Bacilli from different cases are of different 

 degrees of virulence, and bacilli from different animals vary 

 still more. The instructive papers upon "A Comparative 

 Study of Bovine Tubercle Bacilli and of Human Bacilli 

 from Sputum," by Theobald Smith,* and upon "Varia- 

 tion in Virulence of the Bacillus Tuberculosis in Man," by 

 Lartigau,f deserve careful study. 



Smith found the bovine bacillus, as a rule, much more 

 virulent for guinea-pigs than the human bacillus. Lartigau 

 found much variation among bacilli secured from the lesions 

 of human tuberculosis. The virulence was tested by em- 

 ploying cultures only for inoculation, and taking of each 

 bacillary mass exactly 5 mg. by weight, suspending it in 5 c.c. 

 of an indifferent fluid until the density was uniform and 

 the microscope showed no clumps, and injecting into 

 rabbits and guinea-pigs, pairs of animals being injected in 

 the same manner, with the same material, at the same 

 time, and being subsequently kept under similar conditions. 



* "Journal of Experimental Medicine," 1898, vol. in, p. 459. 

 f "Journal of Medical Research," vol. vi, No. 1; N. S., vol. I, No. 

 1, p. 156, July, 1901. 



