PREVENTIVE INOCULATION. 119 



period during which the protective effect of vaccination lasts extends 

 over from three to seven years. 



Vaccination very rapidJy displaced inoculation, and spread to every 

 part of the civilized world. The results have been dwelt upon in 

 innumerable books and pamphlets. At present great outbreaks of 

 smallpox have become very rare, at least in the civilized part of the 

 world, and there is a tendency to forget or ignore the devastations they 

 used to cause. 



The first successful attempt in extending the system of inoculation 

 to other diseases was made only after the discovery of the fact that 

 'infection' is caused by living animal or vegetable parasites, capable in 

 the majority of cases of being cultivated and bred in artificial media 

 outside the animal body. Pasteur found that he was able to effect pro- 

 tection against disease similar to vaccination against smallpox by the 

 use of such artificially bred micro-organisms. 



It may be interesting to relate that this important discovery was 

 made unintentionally, and represents one of those happy 'accidents' 

 which occur to those who diligently search. Pasteur had been working 

 with cultures of chicken-cholera microbes, an extremely fatal form 

 of virus when it is introduced into fowls and small birds. It so hap- 

 pened that one of his cultures was left forgotten in the incubator 

 when work was stopped for the vacation. On the return of Pasteur and 

 his assistants the experiments were continued. When the bottle was 

 discovered, thinking that the microbes might have been exhausted 

 or dead from long starvation, Pasteur tried to make what is called a 

 fresh culture of them, by inseminating a sample from the old bottle into 

 a freshly prepared nutritious broth. The microbes were not dead, and 

 multiplied and grew luxuriantly; but when they were injected into a 

 fowl they caused only a transient and non-fatal disease. To make a 

 fresh start, Pasteur took some old blood, which he had drawn a 

 long time previously from a chicken-cholera fowl and preserved in a 

 cupboard in the laboratory in a sealed-up tube, and made a culture with 

 the material that was in that tube. The culture thus obtained killed 

 fresh fowls as usual, but when it was injected into the bird that had 

 resisted the first culture it resisted this injection also. Pasteur, who 

 excelled all men I ever knew in his ability of quickly analyzing and 

 discerning true connections between facts, required no further hints. 

 Others might perhaps have dwelt on the peculiarity of the fowl that 

 happened to resist the injections, or on some other circumstances. 

 Pasteur relinquished this and other suggestions at once, and thought 

 of the microbe. The fact that old specimens of microbes may 

 become impotent when injected into animals was known to him, 

 and was readily explained by the vitality of such microbes being lowered 



