TUBERCULOSIS. 353 



Even when entering the lungs it may in a similar way be 

 quickly checked in its growth, without the person infected sus- 

 pecting its presence. In other cases the bacilli spread from 

 the first point, and may cause many secondary foci of infec- 

 tion, until finally they distribute themselves through the body 

 and produce numerous tubercles at a distance from the point 

 of original infection. Finally they may get into the blood and 

 be carried over the whole body, producing a generalized tuber- 

 culosis which is quickly followed by death. The disease is 

 thus one of widely-varying virulence, from a slight local infec- 

 tion, which is not noticeable, to a fatal case of generalized 

 tuberculosis. It is a certain fact that the tubercles may be 

 healed, and that many animals, after having contracted the 

 disease as a localized infection, perfectly recover and live out 

 a normal life without further trouble. The importance of this 

 fact in determining the method of treating bovine tuberculosis 

 is evident. 



The identity of the various diseases which now are classed 

 together as of tuberculous origin was not known when the 

 modern study first began. The diseases were long known 

 among animals and various types recognized among men. 

 But that the many forms of the disease, now placed together, 

 all belong to the same type of infection has been demonstrated 

 only in recent years, and this largely from microscopic study. 

 It has been chiefly the discovery of the tuberculosis bacillus in 

 these various infections that has demonstrated their tuberculous 

 nature. 



Variations in Virulence. Not only do different animals show 

 great differences in susceptibility, but different cultures of the 

 bacillus show great differences in virulence. If a series of cul- 

 tures obtained from different sources be tested, side by side, 

 their power of producing disease in slightly susceptible animals 

 is found to vary greatly. The most important aspect of this 

 matter arises from a comparison of the virulence of the bacteria 

 30 



