Whether a soldier in s far off land, a child receiving an oral Polio vaccine dose or an Ontario Hydro working taking tincture of poison ivy to reduce symptoms of contact with the plant the effect is the same - an acquired or enhanced immunity to a chemical poison or living pathogen. Sometimes we confer immunity with an injection, sometimes its taken via the oral route. On both cases it's not uncommon that more than one is required.
When did man begin doing this? "1796" as Wikipedia suggests in 2016 ?
Hardly. In 75ACE the an Epic Poem by Marcus Lucanus documented the practive of an acquired immntiy to snake venom by oral ingestion of successively larger amounts of same. From this the idea "same protects against same" or "like protect from like" came about.
So where in the recored history of man did this emerge? To define the term "history of man" this is a guide from Wikipedia:
Conventionally, it is taken to begin with the earliest-recorded Epic Greek poetry of Homer (8th7th century BC), and continues through the emergence of Christianity and the decline of the Roman Empire (5th century AD). It ends with the dissolution of classical culture at the close of Late Antiquity (300600), blending into the Early Middle Ages (6001000). Such a wide sampling of history and territory covers many disparate cultures and periods. "Classical antiquity" may refer also to an idealised vision among later people of what was, in Edgar Allan Poe's words, "the glory that was Greece, and the grandeur that was Rome."
Thus the timeline looks like, from the zero year on, in ascending order:
75 - Marcus Lucanus first documented immunization by Namib tribesmen. it is impossible to estimate how long the practice has been there, but it's not impossible it could be hundreds if not thousands if years or more.
1541 - Paracelsus, a name chosen to honor the wisdom of the Nero era scientists. defined a axiom: the dose dictates the poison. Father of toxicology, first documented mineral deficiency disease, and invented the spa.
- 1729, the data arbitrarily chosen by other "scholars".