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Frank Fenner was a researcher of viruses whose work also helped establish that smallpox had been eradicated worldwide. He chaired the international commission responsible for certification and presented its conclusion to the World Health Assembly in May 1980. In his firsthand interview, he describes certification as an evidential problem distinct from conducting the campaign itself.^[ref]

The distinction is scientifically useful: recording no recent cases in one place is different from establishing worldwide interruption of transmission. Certification requires examining the coverage and credibility of the observations, not merely counting successful interventions. Fenner also helped assemble a detailed historical account of the campaign, linking epidemiology, vaccination and evidence from different countries. His contribution illustrates how public-health science depends both on changing outcomes and on documenting why a conclusion is justified.^[ref]

Jacques Miller established a central role of the thymus in the development of immunity. In his firsthand account, removing the thymus from newborn mice led to a deficiency of lymphocytes and failure to reject skin grafts. The contrast with removal in adult mice helped reveal a developmental function that adult experiments had obscured.^[ref]

A further comparison separated two consequences of the intervention. In conditions excluding environmental microorganisms, the mice remained healthy but still failed to reject skin grafts. Thus wasting illness and impaired immune responses were not interchangeable observations: illness depended on exposure, while the graft test still revealed the immune defect. Miller's experiments helped replace the idea of the thymus as a largely inactive organ with a developmental explanation for immunological competence.^[ref]