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6 the air; but with all of them there must be conditions favouring their reception and increase. Not only are many persons apparently insusceptible through life to some of these diseases, but all the evidence goes to show that, if the whole population of a country lived under thoroughly healthy conditions as regards pure air, pure water, and wholesome food, none of them could ever obtain a footing, and they would die out as completely as the plague and leprosy have died out, though both were once so prevalent in England.

But during the last century there was no such knowledge, and no general belief in the efficacy of simple, healthy conditions of life as the only effectual safeguard against these diseases. Small-pox, although then, as now, an epidemic disease and of very varying degrees of virulence, was much dreaded, because, owing chiefly to improper treatment, it was often fatal, and still more often produced disfigurement or even blindness. When, therefore, the method of inoculation was introduced from the East in the early part of the eighteenth century, it was quickly welcomed, because a mild form of the disease was produced which rarely caused death or disfigurement, though it was believed to be an effectual protection against taking the disease by ordinary infection. It was, however, soon found that the mild small-pox usually produced by inoculation was quite as infectious as the natural disease, and became quite as fatal to persons who caught it. Towards the end of the last century many medical men became so impressed with its danger that they advocated more attention to sanitation and the isolation of patients, because inoculation, though it may have saved individuals, really increased the total deaths from small-pox.

Under these circumstances we can well understand the favourable reception given to an operation which produced a slight, non-infectious disease, which yet was alleged to protect against small-pox as completely as did the inoculated disease itself. This was Vaccination, which arose from the belief of farmers in