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The important transmission chain is from an infected person's intestinal discharges to another person's mouth, especially through contaminated drinking water, food or hands. In the terminology of 1864, the specific infectious material was associated with the bowel discharges of typhoid fever.

William Budd's 1861 argument makes this a disease propagated from preceding cases. Excreta enter a privy, drain or sewer; contamination can then reach a water supply, food or domestic surroundings and expose another person. Soiled linen and attendants' hands can also carry faecal material into the ingestion pathway. A shared polluted supply can therefore distribute infection to people who have never met the original patient. Removing and safely disposing of the discharges, protecting the supply and preventing transfer to food interrupt successive parts of this chain.^[ref]

This contemporary explanation should be distinguished from the competing suggestion that decomposing organic matter spontaneously generates typhoid fever. A sewer can transmit infection because it receives infected excreta; its smell is not itself proof of the causal material or of a primary inhalation route. The question's historical wording reflects a period of dispute, rather than an obligation to present all competing beliefs as equally correct.

The present explanation identifies Salmonella Typhi and faecal–oral transmission. People who are ill, convalescent or asymptomatic carriers can shed the organism; contaminated food and water convey it to another person. This specific bacteriological account is later knowledge, corroborating the route without attributing modern identification methods to 1864. Typhoid fever must also be distinguished from typhus, which has a different transmission history and biology.^[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.