““Every time someone in Baltimore comes to a public clinic for treatment of syphilis or gonorrhea, John Zenilman plugs his or her address into his computer, so that the case shows up as a little black star on a map of the city. It's rather like a medical version of the maps police departments put up on their walls, with pins marking where crimes have occurred. On Zenilman's map the neighborhoods of East and West Baltimore, on either side of the downtown core, tend to be thick with black stars. From those two spots, the cases radiate outward along the two central roadways that happen to cut through both neighborhoods. In the summer, when the incidence of sexually transmitted disease is highest, the clusters of black stars on the roads leading out of East and West Baltimore become thick with cases. The disease is on the move. But in the winter months, the map changes. When the weather turns cold, and the people of East and West Baltimore are much more likely to stay at home, away from the bars and clubs and street corners where sexual transactions are made, the stars in each neighborhood fade away.””
