In the mid-1700s:
“Smallpox was the scourge of these times, just as the plague had been in the seventeenth century, or leprosy toward the close of the Middle Ages. A medical treatise published in 1774 calls it ‘the most universal disease.’ In France, ninety-five out of every hundred persons contracted it; one out of seven perished as a result.”[1] 
Enter Edward Jenner, who exemplified the best of Enlightenment-era science and medicine in pioneering the practice of vaccination.
In 1796, Jenner intentionally injected a patient with cowpox and then smallpox to demonstrate the effectiveness of vaccination. Subsequent tests were successful, and by 1801 Jenner was confident enough to predict that “the annihilation of the Small Pox, the most dreadful scourge of the human species, must be the final result of this practice.”
Sources:
[1] Claude Manceron, Twilight of the Old Order, Volume One of The French Revolution (A.A. Knopf, 1977), p. 51.
[2] “Jenner and the Eradication of Smallpox.”