62 . INFECTION AND RESISTANCE 



vations regarding naturally acquired immunity and rational experi- 

 mental immunization was made by Edward Jenner. It had been no- 

 ticed before Jenner began his work that milkmaids and others who 

 had contracted cow-pox in the course of their occupations were usu- 

 ally spared when a small-pox epidemic occurred in their community. 

 Sporadic attempts had been made to put this observation to practical 

 use, but no one with sufficient intelligence, persistence, and training 

 had taken up the matter seriously. Jenner, interested by the reports 

 of this nature and by his own observations, was especially impressed 

 by the similarity between the local manifestations of small-pox, cow- 

 pox, and a disease of horses spoken of as "grease." Though at first 

 disinclined to identify small-pox with cow-pox (at present the pre j 

 vailing opinion is that the second is an attenuated form of the 

 former), Jenner thoroughly investigated cases of alleged protection 

 by cow-pox, a claim which before this had been hardly more than a 

 rumor, and finally, with the encouragement of John Hunter, pro- 

 ceeded to the vaccination of human beings with cow-pox, testing the 

 result by subsequent inoculation of the same individual with small- 

 pox. His report to the Royal Society in 1796 and his subsequent 

 publications incorporate the results of these experiments by means 

 of which the practice of vaccination against small-pox was intro- 

 duced and the virtual eradication of the disease from civilized com- 

 munities was attained. 



The principles underlying small-pox vaccination are extremely 

 simple. The attenuated virus 'after inoculation incites a mild and 

 localized form of the disease, from which the subject recovers rap- 

 idly and completely. The recovery implies the mobilization of 

 certain protective forces and a specific physiological alteration of 

 the body in such a way that a permanently, or at least prolongedly, 

 increased resistance against the disease remains. In consequence, 

 if the individual is subsequently exposed to spontaneous infection 

 with this disease, his acquired specific resistance suffices to prevent 

 invasion by the virus. This is merely an artificial imitation of the 

 conditions which obtain when an individual recovers from an attack 

 of a disease and is rendered immune thereby. In this case, however, 

 the attenuation of the virus has eliminated the dangers attendant 

 upon an actual attack. The immunity thus conferred is probably 

 never as perfect nor as lasting as that following a seizure of the 

 disease in its unattenuated form; however, it suffices, as a rule, to 

 prevent spontaneous infection which is never as severe a test as 

 experimental inoculation. 



In contrast to the "Natural Immunity" which is an inherited 

 attribute of race or species, we speak of such increased resistance in 

 a member of an originally susceptible race as "Acquired Immunity." 

 When the immunity has been attained as the result of an attack of 

 the disease itself it is spoken of as "Naturally or Spontaneously Ac- 



