The Snow series is an educational course. We hope you will recognise our efforts by donating to TTE or becoming a paying subscriber, as writing the series took a lot of time and effort.

“The duration of cholera in a place is usually in direct proportion to the number of the population.”

We saw in Lecture 3 that Snow deduced by the presence of an incubation period that the morbid poison of cholera needed time to replicate.  Now, he makes a further deduction (page 56).

Snow deduced that it must resemble a cell as it reproduces itself inside the body. This deduction is pure logic as no one had then connected what we now call Vibrio Cholerae Pacini to the onset of cholera. The vibrios were visible with a microscope, but as we shall see in Lecture 17, the connection between micro-organisms and disease had been made in a faraway land. Lacking modern communication, the two had not been reconciled.

When you make a logical deduction from true premises, the chances of reaching a rational conclusion are high.

With this deduction, Snow aligns with Fracastoro’s contagium animatum theory.

(see Lecture 4).

In On the Mode of Communication of Cholera, 1st edition (MCC 1), published in 1849, Snow proposed two main modes of transmission.

The first was what we now call a propagated source outbreak. This was mainly due to his observation of the closeness of cases with water, either from rivers, harbours or wells. He assumed these were contaminated not from miasmata but from what he called a “poison” or a “morbid poison” specific to cholera.

In MCC 2, he published a table showing the highest mortality among seafarers and longshoremen, presumably due to their contact with contaminated drinking water.

The painting by William Atkins shows the density of vessels in Portsmouth Harbour. Both HMS Victory in the middle ground and the troop transport HMS Serapis on the right are shown departing. The hulks in the background had crews and were used as living accommodations with no sanitary facilities. Everything went overboard, creating a pool of contaminated water.

This was more concentrated, and the poison multiplied when there was little rainfall and in the heat, which explained some of its apparent capriciousness.

CONTEMPORARY THEMES

Contemporary ignorance of the history of discovery and study of respiratory viruses from 1936 to today ensures that distorted and unrealistic messages such as “zero covid” are easily spread. 

Watch the video: Iain Chalmers -- New research should begin with analyses of what is already known.

These have had the catastrophic effect of inducing governments into introducing draconian measures in the “short sharp shock” tradition of “do it and we are done” and a complete distortion of the precautionary principle. One hundred years of discovery and evidence now show us that zero viral circulation of an agent is impossible to achieve, and the draconian measures massively impact society and its weakest, both personally, socially and economically.

The reason why respiratory viruses “move on” as in the descending curve of Farr’s Law of Epidemics (see Lecture 10) is not completely understood. However, Snow’s aphorism cited at the beginning of this lecture holds true. When an agent has run out of susceptibles, its circulation sharply declines in a matter of days or weeks.

Readings

Robert Dingwall. The precautionary principle. Doing stuff ‘just in case’ is not precautionary. You need evidence. Trust the Evidence 14 November 2022.