PLANTS WHICH CAUSE DECAY 379 



stances which neutralize the effect of the toxins by 

 combining with them just as the lime-water combines 

 with the lactic acid produced by the bacteria in milk. 

 The antitoxins may remain in the blood for a long 

 time after the disease disappears, thus making it diffi- 

 cult or impossible for the disease to reappear. This 

 condition is known as immunity, and is familiar to 

 us in the cases where one attack protects against 

 another for a long period, sometimes for a lifetime 

 (smallpox, typhoid, scarlet fever, etc.). 



The principle of vaccination depends on the fact 

 that in the case of smallpox, for example, germs taken 

 from a cow with "cowpox" (a similar disease) may 

 be introduced into the human system and produce an 

 exceedingly mild form of the disease, with the result 

 that the body produces enough antitoxins to give it 

 immunity from the disease for a long period. 



In some cases it has been found possible to obtain 

 the antitoxins from the blood of an immune animal 

 and, by injecting them into another animal (or the 

 human system), confer immunity upon it. 



In addition to antitoxins, there are antibacterial 

 substances (produced in much the same way as the 

 antitoxins) which act, not by neutralizing the effect 

 of the toxin, but by destroying or checking the 

 bacteria. 



By the application of vaccination, antitoxins, etc., 

 we may hope ultimately to conquer such diseases as 



