Jenner didn’t discover that cowpox protected against smallpox, a simple Country Doctor did. The story appears in this week’s The New England Journal of Medicine:
Two hundred fifty years ago, an almost-forgotten country doctor made an observation while inoculating a group of farmers against smallpox. Although John Fewster never appreciated the importance of his discovery, he told his colleagues what he had found, setting in motion a process that led to the development of the smallpox vaccine and the eventual eradication of the virus. All immunizations arguably have their origins in this event.
http://www.nejm.org/doi/full/10.1056/NEJMp1715349?query=featured_secondary