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: