INFECTION AND IMMUNITY. 539 



they are all caused by bacteria, and they all 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 mech- 

 anism of all infections. Light upon this phase of the sub- 

 ject can best be secured through experimental methods. 

 Observation and experiment have taught us that 

 sometimes highly pathogenic bacteria may lose in part 

 or in whole their disease producing properties with- 

 out 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 

 with the fully virulent organism. This is often the 

 case with the bacteria that excite septicaBmia, and the 

 bacillus causing anthrax serves as a useful illustration. 

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

 virulent and causes fatal blood poisoning in susceptible 

 animals, but if subjected to certain chemical and phys- 

 ical 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 blood vessels 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. 



