In 1796, physician Edward Jenner inoculated a young boy with pus extracted from a cowpox sore on a milkmaid's hand. His action not only protected eightyear- old James Phipps from smallpox, but ushered in the era of vaccination. It was a slow start; it would be one hundred years before Pasteur spearheaded the development of vaccines for cholera and anthrax that got things going properly.

Cowpox is principally a disease of cattle, but Jenner noticed that milkmaids, who were liable to get the lesions on their hands, seemed not to be affected by smallpox when it ravaged communities. He was not the first to inoculate with cowpox; a Dorset farmer had safeguarded his family against smallpox twenty years earlier. But Jenner gets the credit for convincing the world. He showed, experimentally, that the recipients of cowpox vaccine were immune to subsequent smallpox inoculation, and published the results leading to its widespread introduction across the world. The word vaccination actually derives from vache, the French for cow. Dahl knew a bit about Jenner, telling me he thought he was ‘a thoroughly good egg’.

In his spare time Jenner experimented with hot-air balloons, and studied nature. Even before his vaccine studies he had become a Fellow of the Royal Society for his work on cuckoos. It was Jenner who described how newly hatched cuckoo chicks are responsible for pushing the eggs and young chicks of their host out of the nest.

‘I didn't know it was Jenner,’ exclaimed Dahl, who had a bit of a thing about cuckoos. ‘Nasty creatures. Did you know they are very particular about which nests they use? It is nearly always the hedge sparrow. I'm not sure why.’

Jenner also showed that cuckoos even have a concave back to help them shift the eggs.

The cowpox virus worked as a vaccine because it is similar enough to smallpox to generate a response that protects against both viruses. The body's initial defence against an infection is the non-specific innate response described earlier for herpes simplex virus: toll like receptors recognise something abnormal, the nucleic acid of a virus, without being sure exactly which virus it is. This is called the innate response, because it is built-in, primed and ready to go. But it is not very strong – only a short-term measure to try to keep the invading microbe under control whilst the full immune defence develops.