56 The Fa/rmer's Veterinary Adviser. 



named is the only one that makes full explanation of the 

 phenomena. 



We can now proceed to intelligently consider the different 

 modes of seeking immunity from contagious diseases. 



I. IMMUNITY BY GOOD HYGIENE. 



We have seen above that the animal system is conquered 

 by any contagious disease in ratio with the debility of the 

 living animal cells and their feeble power of resistance. 

 The system, therefore, in which these cells ai'e weak from 

 living in impure air, damp buildings, darkness, on poor or 

 deficient food, on foul water, from overwork, from old- 

 standing or debilitating disease, from excessive drains on. 

 the vitality, as heavy milking, etc., is more ready to suc- 

 cumb to the attack of a disease-germ than is one in the 

 strength of the most vigorous health. So it is with the 

 individual that has descended from weak or debilitated 

 ancestors, or from such as were too young and imperfectly 

 developed, or too old and worn out. Hence it is that all 

 that contributes to robust health favors the resistance to 

 contagious disease. But this resistance is extremely limited 

 in its scope. We constantly see the strongest and healthiest 

 men and animals fall under the blight of a plague, while 

 their weak and debilitated compeers that have already 

 passed through this affection successfully resist. In many 

 cases, too, the unusual vigor of an animal system, while 

 failing to completely throw off the disease-germ, yet modi- 

 fies the affection so that it passes in a milder form. This 

 may save the individual, but it does not hinder the multi- 

 plication of the germs and the propagation of the plague. 

 The robust system, like a barren field, produces a stunted 

 crop of disease-germs, a crop, liowever, which is amply suf- 

 ficient to keep the contagion constantly progressing from 

 animal to animal, and from herd to herd. 



