24 PROTEIN POISONS 



tion is due to living proteins, we have recently learned 

 that experimental fever can be induced by repeated injec- 

 tions of foreign proteins and by changes in size of dose and 

 in intervals between doses, fever of any desired type can 

 be induced. 



9. Natural immunity to any infection is due to inability 

 of the infecting agent to grow in the animal body. This, of 

 course, does not include toxin immunity, which is due to 

 the presence in the body of an antitoxin or of something 

 which destroys or neutralizes the toxin. The inability of 

 an infection to multiply in the animal body has been 

 explained under 6. 



10. The immunity which is due to recovery from an infec- 

 tion is the result of the development in the body during the 

 course of the infection of a specific ferment which imme- 

 diately destroys the infection on renewed exposure t As has 

 been stated, the cells of the body acquire the function 

 of developing the specific ferment, and this function is 

 awakened and made immediately effective on subsequent 

 exposure. This new function developed in the body cells 

 by disease may continue throughout life, or it may be lost 

 after a period which is variable in different diseases. The 

 immunity induced by one attack of yellow fever is believed 

 to continue through life; that from smallpox generally 

 holds through life, and but few have typhoid fever more 

 than once. Some infectious diseases, such as pneumonia, 

 are apparently not followed by immunity. In most instances 

 it seems that the immunity induced by one attack of an 

 infectious disease is not absolute, but only relative, and 

 may be overcome by severe or prolonged exposure to a 

 virulent form of the infection. 



11. Immunity established by vaccination is similar to 

 that induced by an attack of the disease. The vaccine is the 

 same protein that causes the disease. It must be so modified 

 that it will not induce the disease, but yet so little altered 

 that it will stimulate the body cells to form a specific ferment 

 which will promptly and quickly destroy the infecting 

 agent on exposure. The smallpox virus is modified by 



