Making the case for the water supply

EPID 684
Spatial Epidemiology
University of Michigan School of Public Health

Jon Zelner
[email protected]


  • Syllabus updates

  • General feedback about ‘origin assignments’

  • Snow’s analysis of the Golden Square outbreak

  • Hands-on with the Snow data

Course Updates/Proposal

  • Moving module on multi-level modeling up to next week.

  • Destination assignments now a pre-break (short) presentation.

  • Change ‘postcards’ assignments to be a series of student-run course meetings.

Updated postcards assignment 📬

  • Ask you to assign 1 paper on your topic to the class.

  • Make a short presentation with an overview of the goals of your project, progress so far, and feedback areas from the class.

  • Provide discussion questions about the assigned reading for the class and co-lead discussion w/me.

  • Aim for 2-3 presentations per session over 4 sessions after spring break.

The Pump!

Snow searched for exceptions that prove the rule to convince skeptics

Snow knew that the case would be made in the exceptions to the rule. What he needed now were aberrations, deviations from the norm. Pockets of life where you would expect death, pockets of death where you would expect life. (1, p.140)

Snow’s experience with anesthesia contributed to his miasma debunking

[Snow’s] experience with choloform and ether had also endowed [him] with an intuitive grasp of the way that gases disperse in the environment. Ether could be deadly in a concentrated form, delivered directly to the patient’s lungs. But a doctor delivering it, standing a foot away from the patient, wouldn’t feel its effects in the slightest, because the density of ether molecules in the air dropped at a precipitous rate the farther removed one was from the inhaler itself. (p.145)

In pairs/threes: What happened, exactly, in Golden Square?

  • What was the most likely route by which cholera was introduced to the Broad Street Pump?

  • Why didn’t the outbreak spread exponentially?

  • What is the most plausible explanation for why the outbreak ended?

  • How did Snow think about and use information about distance to flag the pump as a key source of risk?


Put the mechanism on the map

“In his mind snow was already drawing maps. he’d imagined an overview of the Golden Square neighborhood, with a boundary line running an erratic circle around the Broad Street pump. Every person inside that border lived closer to the poisoned well; everyone outside would have had reason to draw water from a different source.” (p. 141)
  • 10 cases outside the boundary.
  • 2 were tailor & son on Cross st.
  • 3 were children who went to school near the pump.
  • 3 more regularly drank from the pump.
  • 2 more could be chalked up to the typical toll of Cholera in a given weekend.

Scale, scale, scale (again!)

“If the symptoms of the cholera concentrated around the small intestine, then there must be some telltale characteristic in the eating and drinking habits of cholera victims. If cholera was waterborne, then the patterns of infection must correlate with the patterns of water distribution in London’s neighborhoods.” (p. 148)

Strength of Snow’s explanation rested on nuts and bolts analysis.

  • Cesspool in the basement of 40 Broad was the source of the outbreak.

  • Others got sick in 40 Broad over the course of the week, so why did they not add additional fuel to the 🔥?

  • Only the Lewis family had access to the basement cesspool, while others threw their waste out the…🪟.

Modern tools for an old problem

Next Time


Johnson S. The Ghost Map: The Story of London’s Most Terrifying Epidemic–and How It Changed Science, Cities, and the Modern World. Reprint edition. London: Riverhead Books; 2007.