Bradley's greatest medical challenge while in Siam was attempting to produce a vaccination for the smallpox virus, which devastated the country and killed Bradley's eight-month-old daughter, Harriet. Bradley received trial vaccines from Boston, none of which were successful. Bradley solved this problem by using the inoculation technique. (The inoculation technique was documented as having a mortality rate of only one in a thousand. Two years after Dr Peter Kennedy's description appeared, March 1718, Dr. Charles Maitland successfully inoculated the five-year-old son of the British ambassador to the Turkish court under orders from the ambassador's wife Lady Mary Wortley Montagu. Four years later he introduced the practice to England.) Seeing the success that Bradley had with the technique, the royal court called on Bradley to vaccinate their children as well as many natives and slaves.