In 1796, English physician Edward Jenner had a hunch. As hundreds of thousands around him succumbed to the blistering agony of smallpox, Jenner’s eye was on the rosy, unblemished complexion of Sarah Nelmes, a local milkmaid. Her skin bore none of the oozing lesions that signaled almost certain death for a third of smallpox sufferers—save for her hands, busily milking a cow named Blossom.
The locals called the unsightly affliction “cowpox.” But apart from a smattering of welts, Sarah and her fellow milkmaids were remarkably free of disease. To Jenner, this wasn’t a coincidence. Acting on little more than sparse observations, Jenner decided to extract a small sample of Sarah’s pus and inject it into the arm of a young boy named James Phipps. To everyone’s amazement—including Jenner’s—when Jenner stuck Phipps with a second needle, this time sporting a hefty dose of smallpox, Phipps remained healthy. Against all odds, the risky treatment had granted the child miraculous immunity.
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