Many years later, pouring through the archive at the museum, I found that Dahl had in fact been doggedly corresponding with scientists for years after, to try to discover a link between smallpox vaccination and measles encephalitis.

‘There must be some clue or clues as to the susceptibility of a person to encephalitis from this vaccination business,’ he wrote to Dr John Adams at the University of California's Department of Paediatrics in 1966. ‘It is probably staring us in the face. In what way for example is resistance to encephalitis provided when immunity to smallpox is achieved? My daughter was congenitally immune to smallpox (no vaccination ever took) and being immune to smallpox she was susceptible to encephalitis … has anyone checked up the smallpox vaccination history of recent cases of encephalitis not only for measles but from all sources? There is a clue here I'm certain of it.’

In 1972 Adams sent Dahl drafts of his manuscripts, later published in Science and Journal of the American Medical Association suggesting that vaccinia, the strain of virus modified to make the smallpox vaccine, may become a slow virus and cause encephalitis years after the inoculation. Dahl thanked Adams for a publication that ‘breaks new ground’, but questioned him about ‘one fact important to my mind that is missing from your report’: had the child affected, who had been vaccinated twice against smallpox, had any reaction to the first vaccination? ‘My guess is there was not.’

The next year Adams sent Dahl an article he had written on a child who developed neuromyelitis optica, another inflammatory condition of the central nervous system, years after primary smallpox vaccination. A decade later Dahl was still pursuing the same theory, sending a copy of this neuromyelitis publication to Professor Neville Butler at the University of Bristol, whose new charity he was supporting. Butler's reply describes why he thinks there are difficulties with Dahl's smallpox theory, but he does not come up with any better explanation as to why a virus sometimes causes such severe disease.

Improving Outcomes

Through my work with the Encephalitis Society I know how important it is for families to understand ‘why?’ even if it is too late for their loved one. It helps them come to terms with the disease, and of course may prevent others suffering the same devastating illness.