THE ALLIES OF THE SQUIRREL 431 



Mammals in the War on Disease. Even a brief summary 

 of the facts regarding the importance of mammals to man 

 must include some mention of the role which mammals have 

 played in the development of modern medicine. By far the 

 greatest advance in our war on disease has been in the direc- 

 tion of the prevention of disease. Much that we now know 

 about human ills has resulted from laboratory studies in 

 which rabbits, guinea pigs, and other laboratory animals 

 have been used in experiments. Further than this, many 

 mammals have aided directly in curing and in preventing 

 human disease by producing serum and antitoxin. 



In the blood of cows the germs of cowpox are produced. 

 These are very nearly like the germs of the dreaded small- 

 pox. When cowpox germs are introduced into the human 

 body by vaccination they cause the blood to become immune 

 to the more dreaded and highly fatal smallpox. 



When the germs which cause diphtheria are injected into 

 a horse, the blood of the horse produces substances called 

 antitoxins. These neutralize the effects of the poisons, or 

 toxins, formed by the germs. The blood of the horse pro- 

 duces more antitoxin than is needed to kill the germs that 

 were introduced into it. When antitoxin from the blood of 

 the horse is injected into a diphtheria patient, the disease 

 is checked. Furthermore, it has been discovered that diph- 

 theria may be avoided altogether. When a mixture of the 

 antitoxin and a small amount of the toxin of diphtheria are 

 injected into the blood of a child, the antitoxin prevents the 

 toxin from injuring the child, but at the same time the toxin 

 causes the blood to produce its own antitoxin. When this 

 toxin-antitoxin treatment is given, the child becomes im- 

 mune to diphtheria. 



In some diseases the dead germs introduced into the hu- 

 man body produce immunity to the disease. The germs of 

 typhoid are grown, or "cultured," in the laboratory. After 



