"Contact Rate," or partner change, describes the average number of sexual partners that people have within a given population or ‘sexual ecosystem,’ such as students at a particular college, or gay men in a particular city. “Simply put, without partner change no STD can spread. Partner A may infect partner B, but things will end there. In a thoroughly monogamous population there would be no STDs at all, no matter how infectious certain microbes might theoretically be. Conversely, the higher the level of partner change, the more likely that even microbes that are relatively hard to transmit will have an opportunity to spread.”Rotello introduces an additional factor in STD epidemiology, the role of ‘core groups.’ He describes these as smaller groups within a larger sexual ecosystem that transmit STDs at significantly higher levels than the surrounding population. This can happen when core members have very large numbers of sexual partners who also have very large numbers of partners, creating a biological feedback loop that can ‘amplify’ diseases that enter the core. If people within the core also have partners outside the core, epidemics can get a foothold within the core and then “bridge” out to larger and larger circles of people.