By this and similar studies was Jenner preparing his acute powers of investigation for the great purpose of his life. For this he secured more time and more extended opportunities for inquiry by abandoning general practice, and confining himself to medicine proper, having obtained, in 1792, the degree of M.D. from the University of St. Andrews. In conjunction with the "dear man," as he used to call his great master, John Hunter, he carried on his experiments illustrative of the structure and functions of animals. With great industry and ingenuity he explained some of the unaccountable problems in ornithology; he ascertained the laws which regulate the migration of birds; made considerable advances in geology and in our knowledge of organic remains; he amended various pharmaceutical processes; he was an acute anatomist and pathologist, and investigated and explained one of the most painful affections of the heart, and many of the diseases to which animals are liable. By such labours he established a just claim to distinction as a medical philosopher, apart from his claims to the gratitude and admiration of mankind by his self-denying and devoted labours in connection with his great discovery; but like other great men absorbed in the establishing of important truths, he was regardless of personal objects, and never ostentatiously promulgated his claims to public distinction.
It was while still a youth, living with his master at Sudbury, that his mind first became deeply impressed on the subject of the cow-pox. A young country woman came to seek advice, when the subject of small-pox was incidentally mentioned in her presence, and she immediately observed, "I cannot take that disease, for I have had cow-pox." This was a popular notion prevalent in the district, and not unknown to Jenner, but from this time he never ceased to think on the subject. On coming to London he mentioned it to several persons, and among others to Hunter; but all thought his notion of getting rid of small-pox Utopian, and gave him little or no encouragement. Hunter, however, who never liked to daunt the enthusiasm of inquirers, said, in his characteristic way: "Don't _think_, but _try_; be patient, be accurate." About the year 1775, some time after his return to the country, he first had the opportunity of examining into the truth of the common traditions regarding cow-pox, but it was not until 1780, after much study and careful inquiry, that he was able to unravel the various obscurities and contradictions with which the subject was involved, and in that year he first disclosed his hopes and his fears to his friend, Edward Gardner. His mind seems to have caught a glimpse of the reputation awaiting him, and he felt that, in God's good providence, it "might be his lot to stand between the living and the dead, and that through him a great plague might be stayed."