A vague notion had prevailed among the dairies from time immemorial that this disease was a preventive of the smallpox.

Jenner put the matter to the test of experiment.

Let me quote here his own words: "The first experiment was made upon a lad of the name of Phipps, in whose arm a little vaccine virus was inserted, taken from the hand of a young woman who had been accidentally infected by a cow.
Notwithstanding the resemblance which the pustule, thus excited on the boy's arm, bore to variolous inoculation, yet as the indisposition attending it was barely perceptible, I could scarcely persuade myself the patient was secure from the Small Pox.

However, on his being inoculated some months afterwards, it proved that he was secure."(8) The results of his experiments were published in a famous small quarto volume in 1798.( *) From this date, smallpox has been under control.
Thanks to Jenner, not a single person in this audience is pockmarked! A hundred and twenty-five years ago, the faces of more than half of you would have been scarred.

We now know the principle upon which protection is secured: an active acquired immunity follows upon an attack of a disease of a similar nature.