326 ANIMAL BIOLOGY 



body conquers the disease and very frequently one attack 

 protects the individual from another attack of the same 

 malady. Typhoid, scarlet fever, smallpox, are rarely 

 taken more than once, the patient having acquired what 

 is called immunity to these diseases. 



Disease germs produce their deleterious effects by 

 generating some poisonous substance, or toxin. The 

 cells of the body have the property of producing substances 

 which neutralize or destroy these poisons, and these 

 are called antitoxins. The white phagocytes of the blood 

 also attack and devour disease-producing bacteria. A 

 disease in the body means a battle between the bacteria, 

 which tend to live and multiply at the expense of the 

 organism, and the cells of the body with their antitoxins 

 and phagocytes. The discovery that bacterial poisons 

 may be destroyed by antitoxins generated by the body 

 has led to efforts to control diseases by injecting anti- 

 toxins into the blood. One of the first and most note- 

 worthy attempts of this kind is the antitoxin treatment 

 of diphtheria. This disease formerly had a high death 

 rate. The rod-like bacilli occur chiefly in the throat 

 and generate a toxin of extreme virulence. The German 

 bacteriologist, von Behring, found in 1892 that if diphtheria 

 toxin is injected into a horse, the blood serum of the animal 

 will contain a substance that neutralizes the toxin. In 

 the manufacture of diphtheria antitoxin healthy horses 

 are given several injections of diphtheria toxins of gradually 

 increasing strength. After several months some of the 

 animal's blood is removed and the serum preserved for 

 injecting into human beings. The antitoxin treatment 

 of diphtheria is now regularly employed and it has reduced 

 the death rate of this dreaded disease by 75 to 80 per cent. 



Rabies, or hydrophobia, has been successfully treated 

 by a method somewhat different from the preceding, 



