What you're looking at is a sexual exposure map. To create it, scientists from Ohio State University tracked the sexual activity of an unnamed public high school in the Midwest. Their findings were published in the July 2004 issue of American Journal of Sociology.
832 of the approximately 1000 students were interviewed, and asked to identify their romantic sexual partners over the past 18 months. A "romantic sexual partner" was defined as someone you were dating, and therefore mere "hook ups" were not included in the research. The research also did not include partners from other schools.
Here's what they found: 573 students admitted to having at least one romantic sexual partner in the last 18 months. Of these 573 students, more than half could be traced to a network of 288 partners! (See image) The pink dots represent actual high school girls, and the blue represents the boys. The lines between them represent the links of sexual activity. As you can see, while a guy may have only one sexual partner, theoretically he could be connected to 286 sexual partners other than his own. In fact, the furthest two people on the map are separated by 37 steps. Surely, not one student in this map would have suspected this intricate web of sexual exposure, or the massive implications this has on STD transmission.
The above image does not include the other 285 students who had been sexually active. These students were involved in numerous and separate smaller networks.
What can be learned from this is that a person who has had only one sexual partner may sometimes be more at risk to acquire an STD than a person who has had multiple sexual partners, but is in a smaller sexual network. It also explains the rampant transmission of STDs among adolescents, and the dire need for chastity education.