LABORATORY RESEARCH, ETC. 271 
Immunity.—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, | 
