the.com/inoculation

means The deliberate introduction of a weakened or harmless form of a pathogen into the body so the immune system learns to fight it before meeting the real threat.

from From Latin inoculare, 'to graft' or 'to implant an eye (bud)' — from in- ('into') plus oculus ('eye,' also used for the bud-eye of a plant). Gardeners used the word for grafting a bud from one plant onto another, long before medicine borrowed it. The leap came in the 1700s, when the term was applied to the practice of 'grafting' a bit of smallpox matter into a scratch on the skin — deliberately planting a small, controlled version of the disease the way you'd graft a bud, so the body would grow its own resistance rather than suffer the full outbreak.