CHAPTER XXV. 

 IMMUNITY. 



Immunity, as has already been stated, implies such a con- 

 dition of the body that pathogenic organisms after they 

 have been introduced are incapable of .manifesting them- 

 selves, are unable to cause disease. The word has taken the 

 place of the earlier term, resistance, and is the opposite of 

 susceptibility. The term must be understood always in a 

 relative sense, since no animal is immune to all pathogenic 

 organisms, and conceivably not entirely so to anyone, since 

 there is no question that a sufficient number of bacteria of 

 any kind might be injected into the circulation to kill an 

 animal, even though it did it purely mechanically. 



Immunity may be considered with reference to a single 

 individual or to entire divisions of the organic world, with 

 all grades between. Thus plants are immune to the diseases 

 affecting animals; invertebrates to vertebrate diseases; cold- 

 blooded animals to those of warm blood; man is immune 

 to most of the diseases affecting other mammals; the rat to 

 anthrax, which affects other rodents and most mammals; 

 the well-known race of Algerian sheep is likewise immune 

 to anthrax while other sheep are susceptible; the negro 

 appears more resistant to yellow fever than the white; some 

 few individuals in a herd of hogs always escape an epizootic 

 of hog cholera, etc. 



Immunity within a given species is modified by a number 

 of factors — age, state of nutrition, extremes oi heat or cold, 

 fatigue, excesses of any kind, in fact, anything which tends 

 to lower the "normal healthy tone" of an animal also tends 

 to lower its resistance. Children appear more susceptible 

 to scarlet fever, measles, whooping-cough, etc., than adults; 

 young cattle more frequently have black-leg than older ones 



