ACQUIRED IMMUNITY 131 



tendency to complications, while others recover without any unto- 

 ward incident, and so on. We must accordingly conclude that some 

 individuals are naturally immune to certain infections and that 

 even among those who are attacked there must be marked quan- 

 titative variations in resistance. 



Acquired Immunity. The different types of immunity which have 

 been briefly considered above have one point at least in common 

 namely, the fact that they exist under natural conditions, and we 

 hence speak of immunity of this order as natural immunity. With- 

 out entering into a discussion of the possible etiological factors 

 which may have been operative in the production of this type of 

 immunity, we may emphasize that it apparently does not depend 

 upon a process of active immunization, viz., upon the introduction 

 either of the pathogenic organism or its product. This is in marked 

 contradistinction to another type of immunity which is directly 

 dependent upon these very factors and which we accordingly speak 

 of as acquired immunity. 



It has long been recognized that individuals who have once passed 

 through certain diseases, such as smallpox, chicken-pox, scarlatina, 

 measles, mumps, whooping cough, typhoid fever, typhus fever, yellow 

 fever, and Asiatic cholera, are subsequently immune either abso- 

 lutely, or to a very considerable extent. The recognition of this fact 

 has been of the greatest importance, for it forms the basis of our 

 modern attempts to create an artificial immunity to different dis- 

 eases, or if not an actual immunity, then at least increased resistance 

 by the purposeful introduction of the corresponding infecting agent 

 in such form as not to expose the individual to the dangers of natural 

 infection. We may thus distinguish between an artificially acquired 

 immunity and what we may appropriately term accidentally acquired 

 immunity. 



The discovery of the possibility of producing immunity artifi- 

 cially we owe to Jenner, who first showed that by "vaccinating" 

 individuals with smallpox virus which had been attenuated by 

 passage through cattle, protection against the dreaded malady 

 could be secured (1798). Although the causative agent of smallpox 

 was unknown, Pasteur subsequently recognized that the principle of 

 vaccination lies in the production of the disease in an attenuated form. 

 The thought hence suggested itself to him that the same principle 

 might be adapted to the prevention of bacterial diseases also, and by 



