268 BACTERIOLOGY 



caused by bacteria, and they all may terminate fatally. 

 On the other hand the clinical symptoms and the pathological 

 lesions are such as to characterize each as a pathological 

 entity. But, as has been intimated, there is a fundamental 

 factor common to all, and the discovery of this factor gives 

 the clue to the true mechanism of all infections. Light upon 

 this phase of the subject can best be secured through experi- 

 mental methods. 



Observation and experiment have taught us that some- 

 times highly pathogenic bacteria may lose in part or in 

 whole their disease producing properties without at the same 

 time losing their vitality. If such "attenuated" bacteria 

 be injected into susceptible animals the result may be 

 nothing; or it may be a modified lesion totally dissimilar 

 to that following injection of the fully virulent organism. 

 This is often the case with the bacteria that excite septicemia, 

 and the bacillus causing anthrax serves as a useful illustra- 

 tion. When normal, as it is usual to regard it, it is fully 

 virulent and causes fatal blood poisoning in suceptible 

 animals, but if subjected to certain chemical or physical 

 influences the virulence may gradually be lessened until 

 finally we may have a living anthrax bacillus that has been 

 deprived of almost all its disease producing power. If 

 animals be inoculated with such attenuated anthrax bacilli 

 the conditions found may be in striking contrast to those 

 produced by the normal germ. Instead of the bloodvessels 

 being almost packed with bacteria, they may contain few 

 or none, and the only bacteria to be found in the body in 

 numbers are at and immediately about their point of deposit. 

 Yet these animals exhibit clinical symptoms and occasionally 

 die. 



Similarly, in other varieties of septicemia, the so-called 



