This map represents 18 months of hookups and breakups at a Midwestern high school from “a virus’s point of view,” says Peter Bearman, director of the Institute for Social and Economic Research and Policy at Columbia University. Boys are in blue, girls are in pink, and they are connected by the possibilities for viral transmission. The map contains no direct links—dark blue lines indicate that two students can infect each other through shared lovers, while gray arrows represent one-way transmission.