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1832 on the first occasion, has exhausted all, or nearly all, of some peculiar unknown organic ingredient in our systems, which is absolutely requisite for its support.

Every individual afflicted with small-pox, scarlet fever, or any of the other diseases above mentioned, is, according to the germ theory, to be looked upon as a sort of hot-bed or forcing-house for the seeds, or spores (as they are called), of that malady.

From his or her body are continually given off in all directions, by the skin, the breath, the perspiration and the other secretions, millions of spores so minute that 20,000 placed end to end would not measure 1 inch in length, and a group of them the size of a grain of sand might contain 50,000,000. Each one of these infinitely minute organisms, if it were received into a human system under favourable circumstances, would rapidly reproduce itself, and after a few days or weeks, corresponding, as already mentioned, to the period of incubation, give rise to a new case of disease—again a new hot-bed for other unprotected persons.

But these spores (like the seeds of larger noxious weeds, which, when allowed to gain a foothold in our fields and gardens, propagate themselves with such immense rapidity) can only develop if they meet with air, moisture, and soil suited to their peculiar requirements. That is to say, if the contagion of small-pox is not carried by the air to un-vaccinated persons until it has lost its vitality, or if the microbes of this loathsome disease do not fall upon good ground, then, and then only, no harm is done to mankind.

It must be remembered that small-pox and other contagious maladies do not arise, as is often supposed, without previous exposure to the seeds of disease. It may be, and doubtless is, frequently impossible to say how certain cases of infectious disease have arisen; but most persons competent to judge are agreed that, in our own day at least, every new case of contagious disease is the immediate offspring of a preceding case.

This truth was clearly demonstrated in an epidemic of measles which appeared last century in the Faroe Islands, an isolated group in the North Sea. For sixty-five years the inhabitants of these islands had been free from measles, when, on April 1, 1846, a workman from Copenhagen, who had arrived three days before, fell ill with the complaint. His two most intimate friends were next attacked, and from that time the malady was traced by Dr. Pannum, the Danish Commissioner, from hamlet to hamlet, and from island to island, until 6,000, out of a total population of 7,782, had been affected by it. Age brought no immunity from the disease, though it was found to spare all who, in their childhood, had suffered at the time of the previous epidemic, more than sixty years before.

Caprieiousness of Contagion.—Contagion is often very capricious. Occasionally, in a family of children, one will be very ill with scarlet