He used it in 1798 in the title of his Inquiry into the Variolae vaccinae known as the Cow Pox, in which he described the protective effect of cowpox against smallpox.

[5]: 100 [6] In Jenner's time, smallpox killed around 10% of the global population, with the number as high as 20% in towns and cities where infection spread more easily.

In the field of zoology, he was among the first modern scholars to describe the brood parasitism of the cuckoo (Aristotle also noted this behaviour in his History of Animals).

[7] In 1770, aged 21, Jenner became apprenticed in surgery and anatomy under surgeon John Hunter and others at St George's Hospital, London.

Returning to his native countryside by 1773, Jenner became a successful family doctor and surgeon, practising on dedicated premises at Berkeley.

In 1792, "with twenty years' experience of general practice and surgery, Jenner obtained the degree of MD from the University of St Andrews".

Jenner contributed papers on angina pectoris, ophthalmia, and cardiac valvular disease and commented on cowpox.

[13] Having observed this behaviour, Jenner demonstrated an anatomical adaptation for it – the baby cuckoo has a depression in its back, not present after 12 days of life, that enables it to cup eggs and other chicks.

[14][15] "The singularity of its shape is well adapted to these purposes; for, different from other newly hatched birds, its back from the scapula downwards is very broad, with a considerable depression in the middle.

Jenner's understanding of the cuckoo's behaviour was not entirely believed until the artist Jemima Blackburn, a keen observer of birdlife, saw a blind nestling pushing out a host's egg.

Not only did he have a profound understanding of human anatomy due to his medical training, but he also understood animal biology and its role in human-animal trans-species boundaries in disease transmission.

While Johnnie Notions had great success with his self-devised inoculation[24] (and was reputed not to have lost a single patient),[25] his method's practice was limited to the Shetland Isles.

[28] By 1768, English physician John Fewster had realised that prior infection with cowpox rendered a person immune to smallpox.

[29][30] In the years following 1770, at least five investigators in England and Germany (Sevel, Jensen, Jesty 1774, Rendell, Plett 1791) successfully tested in humans a cowpox vaccine against smallpox.

[31] For example, Dorset farmer Benjamin Jesty[32] successfully vaccinated and presumably induced immunity with cowpox in his wife and two children during a smallpox epidemic in 1774, but it was not until Jenner's work that the procedure became widely understood.

[37][38] Donald Hopkins has written, "Jenner's unique contribution was not that he inoculated a few persons with cowpox, but that he then proved [by subsequent challenges] that they were immune to smallpox.

[40] Some of his conclusions were correct, some erroneous; modern microbiological and microscopic methods would make his studies easier to reproduce.

[41] The expedition was successful, and Jenner wrote: "I don't imagine the annals of history furnish an example of philanthropy so noble, so extensive as this".

In 1808, with government aid, the National Vaccine Establishment was founded, but Jenner felt dishonoured by the men selected to run it and resigned his directorship.

In 1821, he was appointed physician extraordinary to King George IV, and was also made mayor of Berkeley[2] and magistrate[5]: 303  (justice of the peace).

He continued to investigate natural history, and in 1823, the last year of his life, he presented his "Observations on the Migration of Birds" to the Royal Society.