John Snow’s spot map of cholera victims is often said to be the founding script of epidemiology. Once again, in Tufte’s sense, it is a masterpiece of information design which through its clarity of presentation and the commitment of its author still recommends itself to us. In 1854, London, with a population in excess of 2 million, was the largest city on the planet. It was also knee-deep in human excrement, a situation actually worsened by the invention of the water closet which, in the absence of a functioning sewerage system, simply led household effluent into the cesspools maintained in the basement of most urban dwellings. In the cholera epidemic that affected the city in the summer of that year the content of some of these cesspools infiltrated the drinking water supply in Soho. Poor cistern maintenance was partly to blame: the rapid expansion of the city had made it uneconomical to empty cesspools and cart the waste out as fertiliser to the ever more distant farms around the city. Globalization had also played its part: the novel availability of South American guano on the global market made emptying family cesspools (of which there were more than 200 000) even less attractive. John Snow and Henry Whitehead, a local clergyman, spent days walking Soho. Snow mapped the epidemic, house by house, victim by victim, in his search for evidence. The theory of the day was that the miasma that insinuated itself through the streets of London was the source of disease. Snow thought differently, and although he did not understand the actual mechanism of transmission, his hunch that the water supply was to blame, and the meticulous way in which he plotted the results of his survey — with its cluster of deaths near the water pump at 40 Broad Street — led him, deductively, to know what to do. He petitioned the local authorities to remove the pump’s handle. This was done and the number of cholera deaths, already on the decline, further diminished. A decade later Joseph Bazalgette’s sewerage system, although based on a false premise (‘all smell is disease’), proved to be one of the greatest of all Victorian building projects: it is still in use.