LABOEATOEY EESEAECH, ETC. 271 



Immwnity. — This term is applied to the property 

 which the organism may acquire of being safe from 

 attacks of certain diseases due to microbes, either in 

 consequence of a former attack, or from a condition 

 which doubtless arises from absorbing the pathogenic 

 poison in minute doses, often repeated. Acclimatization 

 frequently constitutes immunity. Thus, in countries 

 where malaria, yellow fever, etc., prevail, the inhabi- 

 tants are less apt to contract the disease than 

 strangers. Such immunity is not absolute, and may 

 be lost in course of time. This has been ascertained 

 in the case of small-pox, so that it is prudent to be 

 revaccinated every ten or twelve years. 



