116 MICROBES AND TOXINS 



Small-pox and vaccination are without doubt of common 

 origin ; Jennerian vaccination is based as much on their 

 relationships as on their differences. In the same way the 

 distinction between the human bacillus and the bovine bacillus 

 is not inconsistent with a common origin. The numerous 

 experiments which have been made to vaccinate cattle with 

 the human bacillus prove that it is a question of two adapta- 

 tions from the same type, and unfortunately they are not 

 sufficiently differentiated for one to be sure that the bovine 

 bacillus is incapable of producing in man a tuberculosis, not 

 merely local and benign, but general and fatal. 



The specificity of bacteria is only relative, but it suffices for 

 the carefully performed bacteriological diagnosis which is so 

 useful in medicine. Instead of the solitary types which once 

 seemed to constitute the whole species, we know now varieties 

 and families of which one member only may be the constant 

 cause of a definite disease ; and this is of advantage to both 

 medicine and hygiene. In theory it is evident that the only 

 point of view to be accepted is that of the plasticity of 

 microbial species, i.e., the Darwinian theory. 



Virulence. — Virulence is in the first place the capacity in 

 a microbe to settle and develop in the bodies of animals; 

 secondly, its capacity of secreting its toxic substances. Even 

 in the strictly toxic diseases, such as tetanus, the intoxication is 

 not the whole malady ; there is a preliminary, the penetration 

 of the microbe, which may or may not find suitable conditions 

 for its growth. 



Virulence is a variable property, and it was in connection 

 with modifications in virulence that Pasteur had the intuition 

 of the possibility of attenuating a virus. 



Diminution of Virulence. — In cultures on artificial 

 media in the laboratory the virulence diminishes spontaneously; 

 the media may be improved by adding animal fluids (serum, 

 blood, ascitic fluid). A little too much or too little acidity or 

 alkalinity, too much or too little peptone or salt, may cause 

 our nutrient broth to lower the virulence of the strain from 

 the change in its reaction. 



The diphtheria bacillus and the streptococcus are not suited 



