Probably introduced into Europe by the Arab invasions of the first century, smallpox was already endemic in Asia at that time. It was the Chinese, in the eleventh century who tested the first immunization technique by placing a person to be immunized in contact with suppurating matter taken from the pustules of a smallpox victim. Though the results were variable (1 to 2% of inoculees died of the disease) this "variolation" technique gradually spread along the Silk Road. It reached Europe in the early eighteenth century and was taken up by European doctors.

Edward Jenner's discovery

In the late eighteenth century, Edward Jenner, an English physician, observed that dairymaids infected by cowpox, a disease caught from cows with symptoms very similar to those of smallpox, then became immune to smallpox. He concluded that cowpox provided protection against the disease.
In 1796, he drew pus from the hand of a woman with cowpox and used it to inoculate an eight-year-old boy. The boy fell ill, but recovered very quickly. Three months later, the child was inoculated with smallpox and showed no signs of infection. Jenner repeated the experiment several times and published his findings in 1798. It was he who coined the term "vaccination", based on the Latin word vaccinus meaning "from cows". It now designates the process of inoculating weakened or dead pathogens to confer immunity to a disease.