It is an August day in the hot summer of 1854. Mrs Lewis of 400 Broad Street is washing her baby's nappy; she empties the water down the drain. Unknown to her, this action will lead to 700 deaths. Her baby has cholera; the drain empties into a cesspit in front of the house; the cesspit is just 3 feet from the public water pump at the corner of the street. Aaron Lai looks at a classic feat of statistical urban epidemiology.