IMMUNITY. 163 



Acquired immunity is illustrated by the protec- 

 tion afforded by one attack of the exanthemata 

 against subsequent attacks. Thus one attack of 

 measles or small-pox, as a rule, affords complete 

 protection. A knowledge of the immunity result- 

 ing in the latter case led to the introduction of 

 inoculation of small-pox prior to the establishment 

 by Jenner of the protective influence of vaccination. 



Immunity may be acquired by acclimatization, 

 for the inhabitants of tropical climates are less 

 susceptible to the diseases of the country, malarial 

 fevers for instance, than strangers. 



In civilised communities also there appears to be 

 a degree of acquired immunity, for the infectious 

 diseases introduced among savages or isolated 

 communities have assumed the most virulent 

 properties. 



The immunity acquired by protective inocula- 

 tion constitutes, in connection with the study of 

 pathogenic micro-organisms, a subject of pre- 

 eminent interest and importance. Pasteur, in his 

 researches upon fowl-cholera, observed that aftor 

 non-fatal cases the disease either did not recur, 

 or the severity of a subsequent attack was in 

 inverse proportion to the seventy of the first 

 attack- It occurred to him to endeavour to obtain 

 the virus of this disease in a form which would 

 provoke a mild attack of the disease, and thus 

 give protection against the virulent form. This 

 attenuation or mitigation of the virus was 



