118 IMMUNITY. 



It is this form of immunity which is responsible for the 

 many conflicting theories of immunity. 



Active form : Years ago, before it even was surmised that 

 diseases might be caused by minute vegetable organisms, it 

 was observed clinically that one attack of some diseases ren- 

 dered the individual secure from a second. It was customary 

 to produce artificial immunity to smallpox by inoculation. 

 A mild case of the disease was selected, and the healthy indi- 

 vidual was inoculated, through an abrasion of the skin, with 

 some of the purulent matter obtained from a smallpox pus- 

 tule. As might be expected, with the conditions prevailing 

 at that time, the person so inoculated often was the means of 

 exciting an epidemic by developing a very severe case of 

 smallpox, and deaths were by no means infrequent. 



This primitive method of inoculation was modified subse- 

 quently by Jenner, who noticed a peculiar affection of the 

 cow's udder resembling smallpox in man. He also noted 

 that the milkmaids contracted this disease, and that afterward 

 they were immune to smallpox. He then inoculated mem- 

 bers of his family with matter obtained from the pustules 

 on a cow's udder, and succeeded in producing immunity to 

 smallpox. This form of inoculation is known as vaccination. 

 The virus is attenuated by its passage through the cow, and 

 its inoculation into man produces simply a local lesion with 

 slight constitutional symptoms, but confers an immunity equal 

 to that following a typical attack of smallpox. 



This form of inoculation is no longer limited to smallpox, 

 but is practised also in anthrax, symptomatic anthrax, cholera, 

 bubonic plague, and typhoid fever. The principle under- 

 lying it is the attenuation of the germ ; and many vaccines, 

 viruses, or attenuated cultures, are now being used to produce 

 these results. 



It is claimed that inoculation with saprophytic bacteria 

 derived from the soil and water will give protection against 

 pathogenic organisms. When a culture of the Bacillus pro- 

 diaiosus is injected into a rabbit sick with anthrax, the animal 

 will recover. 



This form of immunity may be secured by treating the 

 animal to weakened cultures of the germ ; by injecting steril- 



