SECOND ATTACK OF DISEASE. 



227 



into the blood, or they may be forced into it, but they 

 will not excite any changes, or at any rate they will 

 not give rise to disease. The organism which has 

 already been diseased and has recovered, may be after- 

 wards inoculated to any extent, but the living particles 

 will not grow and multiply in it. They will die, or if 

 they do not die they remain in the body without in- 

 ducing any change. The organism which may have 

 been scarcely deranged or nearly destroyed by the first 

 invasion is by that operation rendered proof against 

 a second attack, but this protection is not, at least in 

 many cases, necessarily permanent. After the lapse 

 of a certain period of time, further change occurs, and 

 the organism again becomes fitted for the growth 

 and multiplication of the same kind of disease germs, 

 in fact susceptible to another attack. This period of 

 protection varies in duration, but is probably pretty 

 constant for each particular kind of disease germ. 



Some forms of disease germs not only " protect" 

 the organism from a second invasion of the same kind 

 of germ, but the change induced is an efficient " pro- 

 tection" against allied forms. And there is indeed 

 reason to hope that means may be discovered, per- 

 haps by passing it through the bodies of animals, or 

 in some other way, of rendering the poison milder, 

 without destroying its efficacy as a protecting agent 

 in fact, that we may produce a mild and harmless 

 disease, in order to "protect" the organism from the 

 chance of suffering and being destroyed by a most 



