CONTROL OF BACTERIAL DISEASES 247 



unable to offer resistance to the fungus of bark disease. In 

 that case, unless some specimens can be taken beyond reach 

 of the spores, every chestnut tree in America will be killed. 

 If immune trees can be found, it may be possible to propa- 

 gate from them a strain of immune trees and so save the 

 species to the continent. It is possible, though not probable, 

 that something may be discovered which, injected into the sap 

 of the tree or fed into the tree from the soil, will enable it to 

 resist the fungus, that is, give the tree an artificial or acquired 

 immunity. It is conceivable that we might inject some of 

 the sap from an immune tree into a susceptible tree vacci- 

 nate, or inoculate and so immunize it and save its life. 



Every animal or plant offers some resistance to being eaten 

 alive by a parasite. This resistance may be natural or ac- 

 quired ; it may be mechanical (skin, bark, cuticle, too resist- 

 ant for parasites to break through) or, as is more common, 

 it may be chemical (some poisonous, toxic substance is pro- 

 duced that weakens or kills parasites). As a nation stung 

 by foreign attack begins to make ammunition, so cells of the 

 host may be stimulated by the toxins of a parasite to produce 

 defensive substances antitoxins or antibodies. In this case 

 the acquired resistance, or immunity, is said to be active. 

 If the defensive substance, antitoxin, is injected from some 

 other person or animal, as if a foreign nation sent in its army 

 and ammunition, the immunity conferred is said to be pas- 

 sive, and this is not likely to last so long as active immunity. 

 Recovery from certain diseases (whooping cough, measles, 

 mumps, scarlet fever, smallpox) generally leaves the body 

 armed with acquired immunity against a second attack by the 

 same germs that is, leaves an experienced army that can 

 prevent another invasion. This, in a true sense, is the case, 

 the white blood corpuscles (phagocytes) often gaining the 

 power to eat the germs, probably alive, instead of being 

 eaten by them. The process is not always as simple as this. 



