612 ACTIVE IMMUNIZATION 



munity thus secured with the least danger to life. This practice, how- 

 ever, was not without risk, as the mild disease not infrequently became 

 a virulent one. Later the dose of infectious agent was decreased by 

 applying the virus to a small abrasion on the skin, and the resulting 

 mild but genuine attack of smallpox usually conferred the much-desired 

 immunity. But here again the severity of the disease was not under 

 control, as the virus occasionally assumed increased virulence and in- 

 duced severer infections than were desirable. Finally, Edward Jenner 

 observed and showed experimentally (1796) that when cowpox virus is 

 inoculated into the human being, a trivial infection, since called "vac- 

 cinia," is induced, and that this is followed by an absolute or nearly 

 absolute immunity of many years' duration against smallpox. In other 

 words, the virus, in its passage through the cow, becomes so modified 

 that it can no longer produce smallpox, but is still able to stimulate 

 the production of the specific antibodies against this disease. Jenner 

 worked so hard to establish the truth of this finding that he had little 

 time to devote to the mechanism involved in the process. 



In other words, the work of Jenner was largely empirical, and the 

 explanation was not forthcoming until many years later, when Pasteur 

 laid the basis of scientific immunization by discovering that light, high 

 and low temperature, and exposure could so reduce the virulence of a 

 microorganism that while its injection into an animal was practically 

 without danger or ill effect, it could still stimulate the protective mech- 

 anism of the host and induce a high degree of immunity. 



That this could be done was a fact discovered accidentally by Pasteur 

 in 1879 while working with the organism of chicken cholera. After 

 an absence from home he found . on examining his cultures on his return, 

 that they had become innocuous that hens could bear without any ill 

 effect inoculation of what would formerly have been a lethal dose. The 

 prolonged cultivation of the microorganism had caused its attenuation 

 and Pasteur immediately grasped the far-reaching importance of this 

 discovery. He conjectured that it might be possible to produce a mild 

 and modified form of chicken cholera with a vaccine of the attenuated 

 microorganism which would afford protection to the fowl against the 

 severe form of the disease. This proved to be the case, and established 

 the possibility of so modifying or attenuating the virulence of a virus or 

 germ that, while its administration is not followed by the actual disease, 

 it ts capable of so stimulating the body-cells that the specific antibodies 

 are produced. This discovery formed the basis of prophylactic im- 

 munization or bacterin therapy in general. 



