16 INFECTION AND RESISTANCE 



strains were injected into mice of approximately the same weight as 

 follows: 20 



This example further illustrates another important fact in con- 

 nection with the problem of infection namely, that within the 

 same species of bacteria different races of strains may exhibit widely 

 varying degrees of virulence. This has been known since the days 

 of Pasteur, and it is indeed of great importance in the immunization 

 of animals that weakly virulent strains of a given micro-organism 

 may be used to produce a gradual immunity against the same species 

 of bacteria in their fully virulent condition. Though observed in 

 almost all species of bacteria such variations are especially notice- 

 able in the cases of streptococci and pneumococci organisms in 

 which no two strains may be alike in infectiousness, and in which 

 the injection of some strains into susceptible animals may produce 

 no result whatever, while other strains will kill if administered in 

 the smallest measurable quantities. To a large extent these fluctua- 

 tions of virulence appear to represent degrees of adaptation on the 

 part of the bacteria to the conditions met with in the living body; 

 and the ease with which such variations can often be artificially pro- 

 duced would seem to furnish another proof that the property of in- 

 fectiousness is a biological attribute of relatively recent acquisition. 

 For, although no general statement of absolute accuracy can be made, 

 it is a fairly uniform rule that races of pathogenic bacteria gain in 

 virulence as they are passed through successive animals of the same 

 species, and lose in virulence as they are preserved upon media 

 under conditions of artificial cultivation. 



Further showing this ability to rapidly adapt themselves is the 

 observation that passage through animals of a certain species will 

 enhance the virulence for this species, but often reduce it for animals 

 of another kind. Among the earliest observations on this point are 

 those of Pasteur 21 in his work on rabies. He found that the virus 

 of hydrophobia when successively passed through rabbits gained in 

 virulence until a degree of maximum infectiousness was attained 



20 For making such accurate measurements we have recently found very 

 useful the Precision syringe described by Terry, Jour, of Inf. Dis., Vol. 13, 

 1913. 



21 Pasteur and Thuillier, Compt. rend, de I'acad. des. sc., Vol. XII, 1883. 



