DISEASE, INFECTION, AND EESISTANCE 279 



transmitted to lower animals. Within a certain species, 

 furthermore, certain individuals are found from birth to be 

 much more resistant than other individuals to a particular 

 disease. All of these are examples of natural, that is, con- 

 genital, immunity. 



An immunity, on the other hand, which develops after 

 birth is said to be acquired. It may develop as the result of 

 any one of a considerable variety of conditions. The body 

 itself may take an active part in the development of immune 

 substances, that is, of antibodies. Such an immunity is 

 termed an active immunity. Such, for example, is the 

 immunity which develops as a result of having had small- 

 pox or diphtheria. Immunity of this type, furthermore, 

 results from vaccination, that is, inoculation with dead or 

 attenuated cultures of microorganisms causing the disease, 

 against which it is desired to immunize. The blood serum 

 of an actively immunized animal comes to contain antibodies 

 specific against the disease. 



The blood serum of an actively immunized animal may 

 be injected into an animal which has not been immunized 

 and such an individual may acquire, as a result, a consider- 

 able degree of immunity. In this case, the tissues of the 

 individual into which the serum is inoculated or injected 

 do not play any part in the manufacture of the antibodies, 

 that is, the body is passive. This type of immunity, there- 

 fore, is termed passive immunity. The immunity conferred 

 as a result of the injection of antidiphtheritic serum is of 

 this type, likewise the immunity conferred by the injection 

 of anti-hog cholera serum. 



An acquired immunity may be either permanent or tem- 

 porary. In general an active, acquired immunity is some- 

 what more permanent than a passive acquired immunity. 

 For example, hogs which are passively immunized against 

 hog cholera by the so-called single treatment, that is, by 

 the injection of an anti-hog cholera serum, acquire an im- 



