CHAPTER XXIX 



ACTIVE IMMUNIZATION. VACCINES IN THE PROPHYLAXIS 

 AND TREATMENT OF DISEASE 



WHILE the importance of natural immunity must not be under- 

 rated in the protection it gives us after bacterial invasion has occurred, 

 this immunity is, however, usually relative and seldom absolute, and 

 may afford insufficient protection if the invading bacteria are numerous, 

 or particularly virulent, or if the natural resistance of the organism 

 is weakened by fatigue, disease, or injury. 



Usually the best and most lasting immunity is that actively ac- 

 quired, in which our own body-cells are stimulated or trained, as it 

 were, to produce specific antibodies against the offensive forces of a 

 particular bacterium or other pathogenic agent. A well-marked and 

 lasting degree of this form of immunity usually follows recovery from 

 many of the acute infections, particularly the acute exanthemata, such 

 as smallpox, scarlatina, measles, typhoid fever, typhus fever, etc. In 

 other infections, such as erysipelas, gonorrhea, and pneumonia, the 

 immunity is less complete, of short duration, or entirely absent, and, 

 indeed, a state of hypersusceptibility may actually follow. 



It is very important, in this connection, to remember that the degree 

 of immunity is not necessarily in proportion to the severity of the disease; 

 thus a mild infection may be followed by the much-desired immunity, 

 and while in general there is not considerable protection without infection, 

 the latter does not necessarily imply that a virulent infection, or even 

 the actual disease itself, is present, for discoveries have shown that an 

 active immunity may be acquired by inoculation with the antigen so 

 modified or attenuated that it can stimulate the production of specific 

 antibodies without producing the disease or otherwise greatly disturbing 

 the health of the individual. 



Historic. The facts here detailed are well illustrated in the history 

 of vaccination in smallpox and the development of vaccine therapy 

 in general. Hundreds of years ago the people of the eastern countries 

 were accustomed to expose their children to a mild case of smallpox in 

 order that a similar mild infection might be acquired and a lasting im- 



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