The 1855 Map That Revolutionized Disease Prevention & Data Visualization: Discover John Snow’s Broad Street Pump Map

No, he didn’t help defeat an implaca­ble zom­bie army intent on wip­ing out all life. But Eng­lish obste­tri­cian John Snow seems as impor­tant as the sim­i­lar­ly-named Game of Thrones hero for his role in per­suad­ing mod­ern med­i­cine of the germ the­o­ry of dis­ease. Dur­ing the 1854 out­break of cholera in Lon­don, Snow con­vinced author­i­ties and crit­ics that the dis­ease spread from a con­t­a­m­i­nat­ed water pump on Broad Street, lead­ing to the now-leg­endary info­graph­ic map above show­ing the inci­dences of cholera clus­tered around the pump.

Snow’s per­sis­tence result­ed in the removal of the han­dle from the Broad Street pump and has been cred­it­ed with end­ing an epi­dem­ic that claimed 500 lives. The Broad Street pump map has become “an endur­ing fea­ture of the folk­lore of pub­lic health and epi­demi­ol­o­gy,” write the authors of an arti­cle pub­lished in The Lancet. They also point out that, con­trary to pop­u­lar retellings, the “map did not give rise to the insight” that the pump and its germ-cov­ered han­dle caused the out­break. “Rather it tend­ed to con­firm the­o­ries already held by the var­i­ous inves­ti­ga­tors.”

Snow him­self pub­lished a pam­phlet in 1849 called “On the Mode of Com­mu­ni­ca­tion of Cholera” in which he argued that “cholera is com­mu­ni­cat­ed by the evac­u­a­tions from the ali­men­ta­ry canal.” As he remind­ed read­ers of The Edin­burgh Med­ical Jour­nal in an 1856 let­ter, in that same year, “Dr William Budd pub­lished a pam­phlet ‘On Malig­nant Cholera’ in which he expressed views sim­i­lar to my own.” Germ the­o­ry had a long, dis­tin­guished his­to­ry already, and Snow and his con­tem­po­raries made sound, evi­dence-based argu­ments for it.

But their posi­tion “large­ly went ignored by the med­ical estab­lish­ment,” notes Randy Alfred at Wired, “and was opposed by a local water com­pa­ny near one Lon­don out­break.” The accept­ed, main­stream sci­en­tif­ic opin­ion held that all dis­ease was spread through “mias­ma,” or bad air. Pol­lu­tion, it was thought, must be the cause. After the pump handle’s removal, Snow pub­lished an 1855 mono­graph on water­borne dis­eases. This was the first pub­lic appear­ance of the leg­endary map—after the removal of the han­dle.

Help­ing to inform Snow’s map, anoth­er inves­ti­ga­tor, parish priest Hen­ry White­head had “con­clud­ed that it was the wash­ing of soiled dia­pers into drains which flowed to the com­mu­nal cesspool that con­t­a­m­i­nat­ed the pump and start­ed the out­break,” writes Atlas Obscu­ra. White­head, a for­mer crit­ic of germ the­o­ry, lat­er point­ed out that the removal of the pump han­dle didn’t actu­al­ly stop the epi­dem­ic, which, he said, “had already run its course” by that point.

Nonethe­less, Snow and oth­er pro­po­nents of the the­o­ry were vin­di­cat­ed, White­head had to admit, and Snow’s inter­ven­tion “had prob­a­bly every­thing to do with pre­vent­ing a new out­break.” The sim­ple, yet sophis­ti­cat­ed data visu­al­iza­tion would lead to rad­i­cal new ways of con­cep­tu­al­iz­ing dis­ease out­breaks, help­ing to stop or pre­vent who knows how many epi­demics before they killed hun­dreds or thou­sands. Snow’s map also deserves cred­it for giv­ing “data jour­nal­ists a mod­el of how to work today.”

It was hard­ly the first or only data visu­al­iza­tion of cholera out­breaks of the time. “As ear­ly as the 1830s,” Visu­al Cap­i­tal­ist points out, “geo­g­ra­phers began using spa­cial analy­sis to study cholera epi­demi­ol­o­gy.” But Snow’s was by far the most influ­en­tial, and effec­tive, of them all. In his TED talk above, jour­nal­ist Steven John­son (author of The Ghost Map:The Sto­ry of Lon­don’s Most Ter­ri­fy­ing Epi­dem­ic and How It Changed Sci­ence, Cities, and the Mod­ern World) tells the sto­ry of how the out­break, and Snow’s the­o­ry and map, “helped cre­ate the world that we live in today, and par­tic­u­lar­ly the kind of city that we live in today.”

Read a Q&A with John­son here; head over to The Guardian’s Data Blog to see Snow’s visu­al­iza­tion recre­at­ed over a mod­ern, satel­lite-view map of Lon­don and the Soho neigh­bor­hood of the famous Broad Street pump; and learn more about Snow and dead­ly cholera out­breaks in the crowd­ed Euro­pean cities of the ear­ly 19th cen­tu­ry at the John Snow Archive and Research Com­pan­ion online.

Relat­ed Con­tent:

Flo­rence Nightin­gale Saved Lives by Cre­at­ing Rev­o­lu­tion­ary Visu­al­iza­tions of Sta­tis­tics (1855)

Napoleon’s Dis­as­trous Inva­sion of Rus­sia Detailed in an 1869 Data Visu­al­iza­tion: It’s Been Called “the Best Sta­tis­ti­cal Graph­ic Ever Drawn”

The Art of Data Visu­al­iza­tion: How to Tell Com­plex Sto­ries Through Smart Design

Josh Jones is a writer and musi­cian based in Durham, NC. Fol­low him at @jdmagness

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