VACCINATION. 165 



immunity. Such light attacks are frequently 

 noted sporadically and in epidemics, while occa- 

 sionally an epidemic is mild in character through- 

 out. Epidemics of benign smallpox occur fre- 

 quently. In these instances it seems probable that 

 the mild character of the disease depends on the 

 low virulence of the strain which causes the infec- 

 tion; and the condition suggests the possibility of 

 artificial attenuation of virulent micro-organisms 

 for the purpose of inducing at will infections of a 

 benign character. 



It might be possible so to modify the virus that vaccination. 

 protection could be established without setting in 

 motion the actual disease even in a mild form. 

 An attenuation of this nature had long been prac- 

 ticed with smallpox virus. Before cowpox was 

 resorted to as a source of vaccine, it had been the 

 custom to inoculate the genuine virus of smallpox, 

 for the purpose of producing immunity. Con- 

 trary to the natural expectation, this method, 

 instead of reproducing severe smallpox, often 

 caused the modified disease called variola inocu- 

 lata. This phenomenon may depend on the fact 

 that the virus finds the skin and subcutaneous 

 tissue an unfavorable medium for the development 

 of virulence; a condition which would be equiva- 

 lent to an attenuation of the microbe. The patho- 

 genicity of the cholera vibrio in animal experi- 

 ments is affected similarly in subcutaneous injec- 

 tions. It is now generally considered that cowpox 

 is smallpox which has suffered a decrease in viru- 

 lence because of its passage through the cow. 

 Consequently, when this weakened virus is planted 

 in the skin of man, where it may undergo further 

 attenuation and produce the mildest possible form 



