Steven Johnson talked about his book The Ghost Map: The Story of London’s Most Terrifying Epidemic and How it Changed Science, Cities, and the Modern World, published by Riverhead. The “ghost map” refers to a map of a city neighborhood created by pioneering British doctor John Snow in 1854. Graphically marking deaths from a cholera epidemic, the map helped pinpoint the source of the disease outbreak and eventually led to reforms in city sanitation and a better understanding of water-borne illnesses. After his presentation, Mr. Johnson answered questions from members of the audience.