CHAPTER III 



INFECTION, SUSCEPTIBILITY, AND IMMUNITY 

 Infection is understood to be the introduction into 

 an organism of a disease-producing germ, followed by the 

 growth and development of the germ, and the production 

 of the characteristic effects of the growth and develop- 

 ment of that germ upon the infected organism. From 

 this it follows that every introduction of a germ into an 

 organism does not constitute infection, inasmuch as 

 disease does not always follow such introduction. Be- 

 fore disease can be produced by the introduction into 

 an organism of any particular germ certain conditions 

 must exist. In other words, the organism must be sus- 

 ceptible to the disease and the germ must be virulent. 

 Susceptibility. By susceptibility is meant the ease 

 or difficulty with which a germ develops upon a certain 

 organism. Thus, children are more susceptible to 

 measles, scarlet fever, and whooping-cough than are 

 adults. 



Immunity. The term immunity is used to describe 

 the condition of absolute lack of susceptibility. If an 

 animal is so constituted that the germ of a disease will 

 not grow upon his tissues or that the toxins of that germ 

 are harmless to the animal, the animal is immune to that 

 particular disease. Thus, man is immune to hog cholera, 



24 



