Disease and Immunity 1 03 



cal germs, his blood reaction easily wipes out 

 the few germs which did gain entrance, and in 

 doing that his system develops slightly its powers 

 of meeting and killing this kind of germ. A 

 little later he is bitten again, and again is inocu- 

 lated, but does not know it. In overcoming this 

 second slight attack his powers of combating this 

 particular germ are still further developed. If 

 the disease is continually present in the neighbor- 

 hood he is continually being attacked, and in 

 continually fighting such attacks he develops his 

 powers of resisting this particular disease until 

 he is fully capable of resisting such attacks, no 

 matter how great they may be. 



Real but unnoticed attacks by disease produc- 

 ing bacteria must be quite frequent. Postmortem 

 examinations show that this is true of tubercu- 

 losis. The system, in fighting such attacks, 

 builds up its powers of resisting that particular 

 disease the same as fighting the virus of cowpox 

 builds up the powers of resisting smallpox. And 

 the same thing must be true in the vegetable 

 world. For example, when phylloxera first at- 

 tacked the American vines, there must have been 

 some cases in which the attack v^as so light as 



