62 INFECTION AND RESISTANCE 



Such methods, though barbaric and eventually unjustified by the 

 naturally high mortality incident upon them, were actually brought 

 to Europe from the East, and for a time practiced in European 

 countries. 



The first great advance which bridged the gap between the obser- 

 vations regarding naturally acquired immunity and rational experi- 

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

 ticed before Jenner began his work that milkmaids and others who 

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

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

 radic 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 cox-pox (at present the prevailing 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, proceeded to the vaccina- 

 tion 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 introduced and the virtual 

 eradication of the disease from civilized communities was attained. 



The principles underlying small-pox vaccination are extremely 

 simple. The attenuated virus after inoculation incites a mild and lo- 

 calized form of the disease, from which the subject recovers rapidly 

 and completely. The recovery implies the mobilization of certain pro- 

 tective forces and a specific physiological alteration of the body in 

 such a way that a permanently, or at least prolongedly, increased re- 

 sistance against the disease remains. In consequence, if the indi- 

 vidual is subsequently exposed to spontaneous infection with this dis- 

 ease, 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 attenua- 

 tion 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 infec- 

 tion which is never as severe a test as experimental inoculation. 



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

 tribute of race or species, we speak of such increased resistance in a 



