6o Causes of Disease. 



same organism from different sources] of the tubercle 

 bacillus ; those obtained by culture from the human body have 

 become almost non-virulent for cattle by their adaptation to man, 

 although some strains are readily transmissible. Avian tubercle 

 bacilli are innocuous for mammals (except rabbits) and conversely 

 it is difficult to infect chickens with human tuberculosis. How- 

 ever, Nocard succeeded, by placing human tubercle bacilli, inclosed 

 in collodion sacks containing bouillon, in the peritoneal cavity of 

 chickens, in so adapting their growth to the avian body that there- 

 after (in later generations of the culture) they became pathogenic 

 for birds as well as men. 



In part, at least, the variability in the virulence of microbes 

 of the same species must be recognized as a reason for the 

 occurrence of either a comparatively mild or a severe course of 

 an infectious disease (formerly called genius epizooticus) as well 

 as for its self-limited termination. For example, mouth and foot 

 disease in some of the epidemics which sweeps the country runs 

 an unusually severe course with high mortality, although under 

 ordinary circumstances it is not a fatal disease, recovery taking 

 place in the course of two weeks. The investigations of Loeffler 

 have shown that the virus of this plague, if inoculated from 

 cow to cow, gradually loses its pathogenic powers, whereas if 

 alternately transmitted from cow to swine in a long series the 

 virulence is maintained or even increased. It is interesting, too, 

 that pathogenic organisms may be influenced in artificial cul- 

 ture outside the body so that their virulence may be either in- 

 creased or diminished {change of virulence by artificial culture 

 methods). Toussaint and Pasteur, who were the first to estab- 

 lish this possibility, have demonstrated that anthrax bacilli, 

 promptly fatal to cattle, sheep, rabbits and mice, may be so 

 attenuated in virulence by artificial culture in bouillon at a tem- 

 perature of 42° C that they produce fatal effects only in mice. 

 Similar facts have since been established in connection with a 

 number of microorganisms capable of artificial cultivation, culmi- 

 nating in efficient methods of prophylactic inoculation (Pasteur). 



This is easily understood when we realize that the attenuated 

 germs give rise to but a mild attack of the infection when inocu- 

 lated, which, however, leaves behind an immunity to the disease. 

 On the other hand, an intensification of the virulence, even of 

 microorganisms ordinarily without pathogenic influence, has been 

 obtained by other cultural methods, as in nutrient media rich in 



