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The word 'vaccination' comes from vacca, the Latin word for 'cow'. This is a poignant recapitulation of the history of vaccines. The first vaccine properly so called had, as its active ingredient, the cowpox virus, a close relative of smallpox that however was much less likely to cause severe, disfiguring or lethal disease. Edward Jenner observed that milkmaids, who were often exposed to cowpox, suffered a relatively mild disease, but would be immune to the much more serious smallpox.
In an experiment that would unlikely pass muster in the modern world, he infected James Phipps, then an 8-year-old, with cowpox. He suffered a mild and transient illness, but when he was later exposed to scabs from a smallpox patient, he proved immune.
Unlike the earlier practice of variolation, which has been practised in late Song dynasty China that sought to induce the cutaneous form of smallpox, variola minor, to protect against the more severe forms of smallpox (variola major), Jenner's vaccination used a less pathogenic virus.
He relied on what would later be called 'antigenic similarity', but which was at the time hardly understood.
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Chris von Csefalvay (Computational Modeling of Infectious Disease: With Applications in Python)