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 sufficient 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 lack of proper nourishment, tend to produce 
disease; while light, dry, well ventilated quarters, 
where the animals are well nourished and have sufficient 
exercise, minimize danger from disease. Direct sun- 
light is a destroyer of many disease germs. 
