How Science Helps the Body 15? 



Animal species differ in their capacity to 

 protect themselves against germs and their 

 poisons. Many bacteria deadly to some of 

 the lower animals are harmless to man, and 

 vice versa. This freedom from danger of 

 special microbic invasion is called immunity. 

 Since it is born with the individual, it is called 

 natural or hereditary immunity. Some of the 

 factors concerned in this we have already 

 considered. 



But there is another phase of immunity 

 which we must look at a little more closely. 

 People who recover from some of the infectious 

 diseases, such as small-pox, measles, scarlet 

 fever, and in a less degree from typhoid fever, 

 diphtheria, and others, are protected for a 

 longer or shorter time from a subsequent 

 attack. Such people enjoy, while it lasts, 

 what is very properly called acquired immunity, 

 an immunity acquired through an experience 

 of the disease itself. Now what is it in the 

 bodies of these people who have successfully 

 weathered an infectious disease which pro- 

 tects them from another attack? If we could 

 answer this question we should evidently be 



