CHAPTER XI 

 SUSCEPTIBILITY AND INFECTION 



SUSCEPTIBILITY, as we have learned, is that condi- 

 tion of the animal organism which permits of the growth 

 and development in its tissues of a disease germ with the 

 production of the characteristic symptoms and lesions 

 of the disease of which it is the cause. 



It is the opposite of immunity. Varying conditions 

 and circumstances produce varying degrees of suscepti- 

 bility. No germ is capable of producing disease in all 

 animals under all conditions. 



Age is often an element in susceptibility, the young of 

 all animals being generally more susceptible to infection 

 than the adult. The increased susceptibility of children 

 to measles, scarlet fever, and whooping-cough is well 

 known. Certain germs are capable of producing disease 

 in some animals and incapable of producing it in others. 

 Frequently the susceptibility of the host depends upon 

 the virulence of the germ. Likewise, the severity of the 

 disturbance produced depends largely upon the virulence 

 of the infecting germ, as well as upon the resistance 

 or susceptibility of the host. Again, the degree of sus- 

 ceptibility depends upon the number of germs which 



