EPID 684
Spatial Epidemiology
University of Michigan School of Public Health

Jon Zelner
[email protected]


  • Meet John Snow, systems thinker

  • What does it mean to take a systems perspective on infection history?

  • How did Snow use different levels of scale to push back on the ☁️miasma☁️ theory of cholera infection?

  • Begin spatial density hands-on explainer

John Snow, pioneering anesthesiologist

“He sits alone in his cluttered flat,🐸 frogs croaking around him, illuminated only by candlelight. After a few minutes tinkering…he fastens the mouthpiece over his face and releases the gas. Within seconds, his head hits the desk. Then minutes later, he wakes, consults his watch through blurred vision. He reaches for his pen, and starts recording the data.”

Making use of evidence across spatial scales is a central theme of Snow’s analysis of the 1854 Cholera outbreak.

How many outbreaks we see is really a function of what scale we’re looking at or care about.

He uses a systems perspective to get a sense of how things work across scales

“The bird’s-eye view of the city, the sense of the urban universe as a system, as a mass phenomenon—this imaginative breakthrough is as crucial to the eventual outcome of the Broad Street epidemic as any other factor.” (1, p.97)

What are the elements of a spatial transmission system?


  • Pathogens 🦠

  • People 🧑‍🤝‍🧑

  • Infrastructure 🏚️


  • Reproduction 🔄

  • Short-scale movement 🚌

  • Long-distance movement ✈️

  • Person-to-person contact 👩‍👩‍👦‍👦

Snow’s sensitivity to patterns across scales allowed him to see things other’s didn’t

Another word for this is consilience

“In science and history, consilience…is the principle that evidence from independent, unrelated sources can”converge” on strong conclusions. That is, when multiple sources of evidence are in agreement, the conclusion can be very strong even when none of the individual sources of evidence is significantly so on its own.”(Wikipedia definition of consilence)

Some questions to discuss in pairs:

  1. Why was miasma a compelling explanation for outbreaks of Cholera in South London?

  2. What aspects of Snow’s life made him an ideal investigator of the cause of Cholera?

  3. What are some modern parallels to the strong hold that miasma theory had on 19th century Londoners?

  4. How did Snow start to use evidence at different spatial scales to push back against the miasmatists?


Snow’s Road to the 1854 Outbreak

Rewind to 1848

Rewind to 1848

Rewind to 1848

I smell a rat 🐀…

“[I]t was stretching matters beyond belief to suggest that the room should suddenly become prone to those poisonous vapors the very day it was occupied by a sailor traveling from a city beseiged by the disease.”

In Snow’s words

“Who can doubt that the case of John Harnold…was the true cause of the malady in Blenkinsopp…And if cholera be communicated in some instances, is there not the strongest possibility that it is so in the others - in short, that similar effects depend on similar causes.”

What happened in the 1849 Thomas street outbreak that drove these ideas home for Snow?

“Whether you looked at the evidence on the scale of an urban courtyard or on the scale of entire city neighborhoods, the same pattern repeated itself: the cholera seemed to segment itself around shared water supplies. If the miasma theory were right, why would it draw such arbitrary distinctions? Why would the cholera devastate one building but leave the one next door unscathed? Why would one slum suffer twice the losses as a slum with arguably worse sanitary conditions?” (Johnson, p.74)

The Grand Experiment

What was the experimentum crucis?

  • Snow took advantage of changes in infrastructure to compare neighborhoods served with contaminated and clean water.

  • Example of a natural experiment that exploits some kind of random variation in exposure to make causal inferences where they would otherwise not be possible.

  • What are some other examples of a spatial natural experiment?

Changes in London water service provided an opportunity for Snow

  • Around 1850, parliament mandated that all water companies move their intake beyond the outflow of the Thames by 1855
  • In south London there were two major water suppliers: Southwark & Vauxhall (S&V) and Lambeth.
  • Lambeth moved the intake up in 1852, but S&V waited until 1855.

Snow’s Water Supply Map

Snow’s map of water suppliers

Conducting the grand experiment

  • Snow divided data into sub-districts of South London organized by water supplier.

  • 12 were served by only S&V, 3 by Lambeth alone

  • 1/100 people died of Cholera in S&V only districts, but none among the > 14K living in Lambeth districts

What made the Grand Experiment inconclusive?

  • 16 sub-districts were served by both suppliers

  • Within these districts, water service was overlapping at a fine scale.

  • Snow went door-to-door…but people living in these districts often had no idea where their water came from.

  • So, he collected water from them and tried to find evidence of the causal agent of Cholera in the sample.


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.