This study is testing a medicine called **pritelivir** for people with a type of **Herpes Simplex Virus (HSV)** infection that doesn't respond well to a common treatment called **acyclovir**. HSV causes sores on the skin, and this study is for people who have trouble fighting infections, such as those with conditions like **HIV** or who have had organ transplants. Participants will take pritelivir tablets once a day, starting with a larger dose to quickly reach the right level in their blood. The study aims to see if pritelivir works better than other treatments like **foscarnet** or **cidofovir**. It will continue until the sores heal or up to 28 days, with a possible extension to 42 days based on progress. To join, participants must be 16 or older, have a type of HSV that doesn't respond to acyclovir, and meet other health criteria. **Key Points:** - Participants need to take daily medicine for up to 28-42 days. - The study includes regular health checks and specific entry requirements. - This trial is for people who have not responded to traditional HSV treatments. **Eligibility and exclusion criteria should be reviewed thoroughly with the study team to ensure fit.**