the matter from an artificially infected animal was transferred by in- oculation 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 in- oculation 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 im- punity 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 re- strictions 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 vac- cinated 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