Contagious Diseases 357 



Vegetable germs are known generally as bacteria or 

 microbes. In many cases they cause disease by throw- 

 ing off, as they multiply and grow in the attacked 

 animal, a definite poisonous chemical substance called 

 a "toxin." If this toxin exists in sufBcient amount 

 and is not neutralized, the animal dies from its poison- 

 ing. In many such diseases, as soon as the germs begin 

 to throw off the poison, certain cells of the animal's 

 body begin to manufacture a chemical substance that 

 neutralizes the poison made by the bacteria ; this sub- 

 stance is called an "antitoxin." If the cells of the body 

 are strong and active, they usually form enough antitoxin 

 to overcome the toxin made by the germs, and the animal 

 recovers. The power of resistance to a disease is called 

 "immunity." Some animals are naturally immune; 

 that is, they do not contract the disease when they 

 are exposed to it. After an animal has had the 

 disease, or has been vaccinated, it often acquires im- 

 munity. 



Whenever a large number of animals are gathered 

 together, the danger from contagious diseases is greatly 

 increased, and extra precaution in the way of hygiene 

 and sanitation must be observed. Anything that weak- 

 ens an animal's system or lowers vitality predisposes 

 that animal to attacks, especially by contagious dis- 

 eases. Dark, damp, filthy, and poorly ventilated quar- 

 ters, and a laelr of proper nourishment, tend to produce 

 disease ; while light, dry, well ventilated quarters, 

 where the animals are well nourished and have suflBcient 

 exercise, minimize danger from disease. Direct sun- 

 light is a destroyer of many disease germs. 



