sipping of coffee and covert crunching of 

 cookies, I was reading John Snow's boolc 

 On the Mode of Communication of 

 Cholera. Snow was a dedicated, lonely 

 workaholic who spent many years during 

 the mid-nineteenth century trying to un- 

 derstand how cholera was transmitted. He 

 focused on a middle-class residential area 

 of south-central London; the northbound 

 Thames bends sharply to the east and then 

 arcs to the south around the area. Cholera 

 battered the residents in 1849. Snow was 

 looking for risk factors: activities or envi- 

 ronmental exposures that could explain 

 why cholera attacked some people and not 

 others. His initial observations made him 

 suspect the water. In one severely affected 

 area he found that 



slops of dirty water, poured down by the in- 

 habitants into a channel in front of the 

 houses, got into the well from which they 

 obtained water.... Owing to something 

 being out of order, the water had for some 

 time occasionally burst out at the top of the 

 well, and overflowed into the gutter or chan- 

 nel, afterwards flowing back again mixed 

 with the impurities; and crevices were left 

 in the ground or pavement, allowing part of 

 the contents of the gutter to flow at all times 

 into the well; and when it was afterwards 

 emptied, a large quantity of black and 

 highly offensive deposit was found... evac- 

 uations [from cholera cases] were passed 

 into the l3eds,...the water in which the foul 

 linen was washed would inevitably be emp- 

 tied into the channel. 



Water in this area was supplied by the 

 Lambeth Company or the Southwark and 



Vauxhall Company. When one of Snow's 

 colleagues examined the water, he "found 

 in it the hairs of animals and numerous 

 substances which had passed through the 

 alimentary canal." He concluded that the 

 water from these companies "is by far the 

 worst of all those who take their supplies 

 from the Thames." 



Before the cholera epidemic of 1853, 

 the Lambeth Company moved its water in- 

 take to a purer source. Snow realized that a 

 vast experiment had been set before him. 

 Scattered among the houses receiving 

 contaminated water from the Southwark 

 and Vauxhall Company were houses re- 

 ceiving purer water from the Lambeth 

 Company. If water transmitted cholera, the 

 residents served by the Southwark and 



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