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Edward Jenner's work on smallpox inoculation is well documented. However long before him the practice was already common in Asia though it was not understood scientifically. Lady Mary Montagu was credited with begining the practice in Europe after observing it in the Ottoman Empire.
But what if this idea was learned by Europeans earlier? Concievably Marco Polo could have picked it up in his travels and publicized it. What would be the historical implications if inoculation became common throughout Euroasia by the late Middle Ages?