Some of these were directed to the treatment of the patient preparatory to inoculation, others to the preparation of the infectious matter in order to attenuate its virulence. The Brahmans, who were the operators in India, in addition to selecting material from patients with a mild form of the disease, were accustomed not to employ the pus at once, but to keep it wrapped up in cotton wool for a period of about twelve months, and thus to weaken its power. They inoculated in the early part of the year, at the time when smallpox prevailed, and the practice they used was to moisten with water a bit of cotton wool prepared in the previous outbreak, to place it on the arm of the person to be inoculated, and to prick the arm, through the wool, over an area of about the size of a twenty-five cent piece. In a few days a vesicle would appear at the seat of the inoculation, which later on developed into a pustule and eruption. Notwithstanding these precautions, great variation in the results was observed, and many succumbed to the operation; but those that passed through it safely were proof against further attacks.

Besides the personal risk to the inoculated, the illness produced in them was infectious to others, and unprotected persons coming in contact with the inoculated were likely to get infected from them. The latter result was largely avoided by the practice adopted by the Brahmans of inoculating all the inhabitants of a family or village at the same time. The benefits secured under the above precautions were considered far to outweigh the risks of inoculation.

With the extension of smallpox westward the system of artificial protection spread toward Europe through the intermediary of travelers and merchants. The Arabs and Turks appreciated its benefits at an early date. The slave dealers supplying the bazaars and harems of Constantinople adopted the system to protect against disfigurement their Circassian and other live stock. In the early part of the eighteenth century the method was made known to the English practitioners by Lady Mary Wortley Montagu, the wife of the English ambassador at Constantinople, who had her two children inoculated according to the Turkish system. Curiously enough, it was soon afterward discovered that a similar method was in practice among the peasants of some of the districts in Wales and the Highlands of Scotland, and had long been known there as ‘buying the smallpox.’ When inoculation was given a more extensive trial it was found, in England as in the East, that the effect of it was decidedly beneficial, but fraught with danger. At first one in every fifty of those operated upon succumbed to the consequences of inoculation. By improved methods the mortality was gradually reduced to one in a thousand; but the most serious danger lay in the spread of infection to healthy persons. The precaution of inoculating whole groups of inhabitants at one time, or of keeping the inoculated apart from the healthy, as had been practiced by the Brahmans ages ago, was overlooked, and the result was often disastrous to the community.

It was at this time that Jenner achieved great progress and threw a vast amount of new light on the question. As is well known, he started from a belief that existed in the west of England, that cowpox was a bovine form of smallpox, and that the milkers who attended on cows suffering from that disease and who became infected with the eruptions on the teats and udders, passed through a mild illness, which rendered them immune against smallpox. Jenner determined to put this tradition to the test, and succeeded in establishing, by a few accurate and well-planned experiments, a series of most important facts.

He showed, first, that cowpox could be artificially given to the cow by infecting it with virus from a smallpox patient, and that the disease thus produced was transferable by inoculation from cow to cow.

He showed further, that by having been bred in the tissues of the cow, the virus lost its intense infective properties for man. When the matter from an artificially infected animal was transferred by inoculation to a human being, it produced at the seat of its insertion a discrete vesicle, which was not followed by a general eruption, as would often be the case with the original smallpox virus.

Though the illness thus induced was not infectious in the sense that it would not be communicated spontaneously from person to person, it could be so transferred artificially by inoculating patients with the lymph from a ripe human vesicle.

When transferred from cow to cow or from man to man the matter preserved unchanged the same property of producing the mild inoculation vesicle, harmless to the patient and to his surroundings; and thus a matter for inoculation was obtained of invariable strength, what was called later on, by Pasteur, ‘virus fixe.’

The last and the most essential property which Jenner demonstrated to belong to the substance in question was the following: A man who had been inoculated with that substance could afterward be with impunity infected with a virus taken direct from a smallpox patient; the inoculation would be either abortive altogether or the effect much milder than in a man not so prepared. Jenner concluded from this most striking result that the inoculation with the matter cultivated by him in the cow would protect a man forever against contamination with smallpox, and he called that matter ‘vaccine,’ or cow lymph.

Jenner’s experiments produced an immense impression throughout the world, and inoculation according to his system, which was called ‘vaccination,’ was rapidly applied to large numbers of people. When outbreaks of smallpox occurred in the midst of vaccinated communities, observations began to come in as to the actual effectiveness of the method in protecting against the disease.

These observations proved that the system possessed an undoubted and exceedingly high beneficial effect, though the following two restrictions had to be imposed upon the originally conceived expectations:

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