period during which the protective effect of vaccination lasts extends over from three to seven years.
Vaccination very rapidly 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 protection 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 happened 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