1. The protection was not absolute. In every outbreak of smallpox a number of patients were and are met with who are attacked, generally mildly, but also in some cases fatally, though they had undergone a successful vaccination, some even at a comparatively recent date before the attack. Only the proportion of such patients to the whole of the vaccinated community is very markedly smaller than the proportion of attacks in the non-vaccinated; and also the severity of the attack, as well as the proportion of deaths to attacks, is in the vaccinated much smaller. 2. This favorable difference between the outbreaks among vaccinated and non-vaccinated is maintained not for life, but for a limited number of years, and disappears gradually, and at length altogether, unless the individuals be revaccinated. Observation has shown that the 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.