A new study helps explain how the virus that causes herpes might change during transmission between partners and over time during a long-term infection within a human host, which could have implications for future treatment strategies. The study, by a team of researchers from Penn State and the University of Washington, is the first to track genetic differences of the herpes simplex virus (HSV-1) as it moves between adult sexual transmission partners. Their findings revealed that viral population movement between partners has a strong impact on whether genetic differences appear at the start of new infections.