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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]

James M. Robins develops statistical methods for causal inference in epidemiology. His joint exposition with Miguel A. Hernán explains how inverse probability weighting can estimate population treatment effects from observational studies under appropriate identifying assumptions.^[ref]

Weighting gives more influence to people whose observed treatment was relatively unlikely given their measured baseline covariates. In the resulting weighted comparison, measured treatment-selection differences are removed under the assumed model. Validity still depends on conditional exchangeability, consistency of potential outcomes and positivity in causal inference; extreme weights also expose a practical difficulty when some treatment choices are rare.^[ref]

For an answer situated in 1867, the important established observations were the association of outbreaks with polluted drinking supplies, the spread from particular cases, and the infectious importance of intestinal discharges. A leaking privy or cesspool near a well, contamination of a stream used for drinking, and household transfer from soiled material to food offered intelligible chains of transmission. Merely saying “defective drainage” names a circumstance; the explanation must show how material from an infected person reaches someone else.

William Budd had already published a particularly clear argument in 1861: the intestinal discharges of sufferers conveyed the specific infectious material, and sewers propagated the disease because they received those discharges. He rejected the claim that inability to trace a previous case established spontaneous generation from putrefaction. This is a contemporary argument that an informed 1867 candidate could have used; it does not require importing a discovery made decades afterwards.^[ref]

The distinction between observation and interpretation matters. Some contemporary writers treated decomposing excreta or “sewer air” as a direct fever-producing agent, under the historical pythogenic theory of typhoid. The observed association of typhoid fever with foul drains did not prove that theory: the same drain could contaminate water, hands or food. Nor should typhoid fever be conflated with typhus, despite the similar names.

In present terms, the causal organism is Salmonella Typhi, a human-associated bacterium. People ill with typhoid fever and people who continue carrying the bacterium after recovery can contaminate food or water. Faecal–oral transmission explains why someone need never enter a sickroom to become infected. The contaminated material may pass through a drinking supply, food handling, or hands rather than by direct contact with the original sufferer.^[ref]

To investigate an outbreak, construct a dated list of cases, compare drinking and food exposures among affected and unaffected households, trace wells and pipes relative to drains, and ask whether cases follow distribution of a common supply. A supply-specific pattern supports a transmission hypothesis more strongly than a general complaint that a neighbourhood smells bad. Modern bacterial culture and organism comparison can supplement this epidemiology, but a modern laboratory identification must not be represented as an accomplished experiment in the 1867 paper.

Skip Thompson is a mathematician whose work includes the numerical solution of delay differential equations. His authored account explains equations in which the present rate of change depends on earlier values of the solution, with applications including population dynamics and epidemiology.^[ref] For example, $y^{\prime }(t)=-ay(t-\tau )$ with $\tau >0$ requires an initial history on $[-\tau ,0]$, rather than only one initial value. A numerical method must therefore approximate and retrieve past values as integration proceeds. Delayed feedback can also create oscillations that are absent from a corresponding equation without delay.

A mathematician.^[ref]