CHAPTER V. 



Healthily Acquired Immunity. 



It is an arresting fact in medicine that some degree 

 of immunity may be acquired by mere contact with 

 cases of an infectious disease without an obvious attack. 

 This may be called healthily acquired immunity.* Thus, 

 the protection of those who have never contracted the 

 disease and yet seem immune in spite of exposure to 

 infection is high where the disease has been rampant 

 previously. Conversely, in "virgin soil," i.e., a com- 

 munity not previously exposed to infection, the 

 ravages have been widespread and excessive, and very 

 few persons have escaped. Quite recently there was an 

 instance of this on the island of St. Kilda, in the Outer 

 Hebrides (Hall, 1913). On the arrival of the fortnightly 

 boat from the mainland an epidemic of influenza broke 

 out and all the inhabitants with the exception of six 

 elderly persons were struck down within a few days. 

 There are many similar experiences in epidemiological 

 records (see articles in Encyclopsedia Britannica) the 

 rapid and terrible spread of " Russian " influenza in 

 1890; the introduction of measles into the Faroe Islands 

 in 1846 (three-quarters of the population being attacked 

 within six months) ; and the similar epidemic in Fiji in 



* Immunity may be inherent; or it may be acquired, either by arti- 

 iicial or by natural means. Immunity may be acquired naturally 

 either in a healthy physiological way (where the patient does not 

 exhibit the disease), or in a pathological way (where the patient suffers 

 from the complaint). 



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