During an urban disease outbreak, health workers collected addresses, dates, symptoms, water sources, and reported deaths using paper forms. A map allowed them to place separate cases into a visible geographic pattern. Clusters near one district suggested a possible shared exposure, but location alone could not prove the cause. Investigators interviewed families, inspected wells and pumps, reviewed population density, and compared affected streets with nearby areas that showed fewer cases. Records were incomplete because some illnesses were never reported and addresses changed as people moved. Officials also faced urgent pressure to act before every uncertainty was resolved. Historians examining the response compare maps with hospital books, newspapers, engineering plans, and later scientific understanding. They distinguish what participants knew at the time from explanations developed afterward. The map mattered because it organized evidence and made a hypothesis easier to test, not because a picture automatically produced truth. Public communication was equally important. Residents needed practical guidance, while authorities had to avoid hiding uncertainty or blaming communities without evidence. Improvements to water and sanitation eventually reduced many risks, though progress occurred unevenly. This episode demonstrates how careful data collection supported public health before digital tools. It also shows familiar challenges: missing information, competing explanations, unequal conditions, and decisions that must balance speed with responsible interpretation.