i iS POPULAR SCIENCE MONTHLY. 



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 



