In this video segment adapted from Rx for Survival, actors portray how John Snow, a London physician, traced a major outbreak of cholera in the 1850s to its source. Using logic, statistics, and mapping, Snow rejected the idea that cholera was carried in a cloud of bad air. Instead, he believed contaminated water was responsible for spreading the disease among the local population. Snow’s surveillance and response tactics would become a foundation of modern epidemiology—the science of public health that is built on a working knowledge of probability, statistics, and sound research methods.