TYPES OF IMMUNITY 507 



is no need of an inoculation, nor any danger of an attack of the dis- 

 ease against which there is the immunity. That there is a natural 

 immunity of species is shown by the innumerable diseases of animals 

 to which humans are not susceptible. Similarly, animals con- 

 tract very few of the human diseases. A natural immunity is, 

 also, evidenced by various groups of the same species, although 

 this is somewhat relative. The natives of South America and 

 Africa are more immune to yellow fever than are the white peo- 

 ple. Perhaps this is due to a weeding-out process during which 

 the fittest has acquired an immunity while the unfit died. This 

 does not always explain natural immunity, however. Measles is 

 a mild disease with white people, but fatal to natives of certain 

 South Sea islands. Jews are more immune to tuberculosis than 

 Irish and English people. Eskimos, Indians, and Negroes are 

 highly susceptible to tuberculosis. The North American Indian 

 seems to be immune to scarlet fever. The colored people in the 

 southern part of the United States seem to have a natural resist- 

 ance to diphtheria. This type of natural immunity is often 

 called racial immunity. Certain members of the same race show 

 a natural immunity to disease while others do not. For example, 

 some children seem naturally immune to diphtheria as seen in 

 their reactions to the Schick test. Whether natural immunity 

 is the result of natural selection, or whether it can be explained 

 by environmental conditions, is still not decided. 



Acquired immunity differs from natural immunity in that it is 

 developed during the lifetime of the individual. There are two 

 types of acquired immunity, active and passive. Active acquired 

 immunity is produced by the body itself as a result of having the 

 germs or the toxins of a disease enter the body. There are three 

 ways in which this may be accomplished. (1) By an actual attack 

 of the disease. If this method produces immunity for any appre- . 

 ciable length of time, it is likely to last for life. For example, once 

 a person has had typhoid, smallpox, or diphtheria he is likely to 



