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.

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1 CROSS-CLIQUE CONNECTIONSBearman expected to see the data describe a traditional “core” model, where small, tightly knit groups of subjects date only each other. To his team’s surprise, 266 students—more than a quarter of the student body—ended up connected to each other. A path linking the two most distant people in the population requires only 37 steps from person to person, dot to dot. That, says Bearman, is a recipe for an epidemic.

2 THE EX FACTOROne of the mechanisms driving the shape of the graph is powered by an unspoken rule of high school dating: Don’t date your ex’s current date’s ex. In other words, if Dylan had been dating Brenda and Kelly had been dating Steve, then after Dylan and Kelly hook up, Brenda and Steve are going to keep their distance. “They’re all watching each other,” Bearman says.

3 BUNDLES OF LOVEDense clusters like this one show that even if one person in a densely connected component began to, say, use a condom, his behavior would effect little positive change. The condom wearer would be protected, but these close-knit bundles represent a viral playground, with the possibilities of infection, reinfection, and re-reinfection piling up with every new member.

4 DEAL BREAKERSA number of individuals join together different clusters of connected lovers, however. If one of these people (they’re called cut points in the sociologists’ study) used a condom and knocked out cross-clique infiltration, he could single-handedly short-circuit the epidemic.

5 HOW CLOSE IS TOO CLOSE?These two blue dots represent two high school males who are probably a lot closer than they think. The solid blue line indicates that they probably had concurrent liaisons with the same girl and are therefore free to pass STDs back and forth. The girls represented by the pink dots at the end of the chain, however, don’t face a risk—the one-way gray arrow suggests that they knew these guys before the love triangle developed.