) 17 



CONTROL OF BACTERIAL DISEASES "24 



unable to ofi'cr resistance — to the fungus of ])ark disease. In 

 lliiil ease, unless sonu^ specimens can l)e taken Itt-ynnd r(»acli 

 of tlie spores, every cheslnnl tree in America will \)v. killed. 

 If innnnne trees can be found, it may be ])ossil)le to propa- 

 gate from tliem a strain ol' inniiune trees and so save the 

 species to the continent. It is possible, tliongli not i)i-o])al)le, 

 that something may be discovered which, injected into the sap 

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

 resist the fungus, that is, give the tree an arlilieial ur aecpiired 

 immunity. It is concHuvable that we might inject some of 

 the sap from an imnuine tree into a susceptible tree — \ aeei- 

 nate, or inoculate — and so inununize it and save its life. 



Every animal or plant offers some resistance to being eaten 

 alive bv a i)arasite. 'J'his 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 anununition, 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 sul)stance, antitoxin, is injected from some 

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

 and anununition, the imnumity conferred is said to be pas- 

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

 Recovery from certain diseases (whooping eough, measles, 

 nnimps, scarlet fever, smallpox) generally leaves the body 

 armed with acquired innnunity against a second attack by the 

 same germs — that is, leaves an experienced army that can 

 prevent another invasion. 'I'his, in a true sense, is the case, 

 the white blood corpuscles (phagocytes) often gaining the 

 power to eat the germs. prol)ably alive, instead of being 

 eaten by them. The process is nut always as simple as this. 



