ACQUIRED IMMUNITY 63 



quired Immunity." When produced by some form of treatment 

 with the virus of the disease, altered in such a way that an actual 

 attack is avoided, we speak of it as "Artificially Acquired Im- 

 munity." 



The premises of Jenner's reasoning were valid as his experiments 

 were convincing. But knowledge regarding infectious disease and 

 its causation by living germs was not developed until almost one 

 hundred years later, by the work chiefly of Pasteur. For this reason 

 no direct continuation of Jenner's work appeared until Pasteur 

 made his communication upon Chicken Cholera to the Parisian 

 Academy of Medicine in 1880. Though his investigations differed 

 entirely from those of Jenner both in method and the nature of the 

 disease with which they dealt, Pasteur recognized the similarity of 

 the fundamental principles underlying both discoveries. 



His observations took origin in a purely accidental occurrence. 

 Cultures of chicken cholera which had been allowed to stand with- 

 out transplantation and under aerobic conditions for periods of sev- 

 eral months were found to have diminished in virulence. Inoculated 

 into chickens, they failed to kill, giving rise in many cases to localized 

 lesions only. It occurred to Pasteur that inoculation with such an 

 attenuated culture might protect against subsequent infection .with 

 fully virulent strains and, indeed, experimental investigation of this 

 idea proved to be correct. He developed a method of "vaccination" 

 against chicken cholera which consisted in injecting first a very 

 much attenuated culture of the organism (premier vaccin), and, 

 after 12 or 14 days, another less perfectly attenuated (deuxieme 

 vaccin), since he observed that a single inoculation was often in- 

 sufficient to confer protection. After two inoculations a degree of 

 immunity could be attained which sufficed to protect against spon- 

 taneous infection as well as against experimental inoculation with 

 doses of the virulent germs, fatal for untreated animals. 



These experiments, simple as they are, constitute the beginnings 

 of the science of Immunity, since, for the first time, an investigator 

 working with a pure culture of a pathogenic micro-organism had 

 succeeded, in planned and purposeful experiments, in conferring 

 artificial immunity. The path was now clearly indicated and the 

 years immediately following were fruitful in the development of 

 many methods by which pathogenic bacteria may be attenuated 

 and changed in such a way that they can be used to confer immunity 

 without causing more than a transient and harmless reaction in the 

 subject. Most of the earlier discoveries of this kind came from 

 Pasteur himself and from members of his school. 



Since in all these methods the inoculated animal attains its in- 

 creased resistance by reason of the activities of its own tissues, these 

 processes are spoken of as "Active Immunization." No protective 

 factor is conferred directly. The disease itself is inoculated, though 



