Investigating and Modelling the Natural History of Dengue in Hospitalised Patients to Improve Future Research

The goal of this observational study is to investigate the natural history of dengue in hospitalised patients in Vietnam, to better understand the disease process, and utilise the data to improve future clinical trials. The main questions it aims to answer are: In participants hospitalised with dengue in Vietnam: 1. How does dengue illness change over time, particularly the development and recovery of vascular leak (where blood vessels leak)? 2. Can a new statistical approaches describe dengue illness accurately and be suitable for use in future clinical trials? 3. Which blood biomarkers are associated with worsening or improving dengue illness, and what do they tell us about how severe dengue develops? Could these blood biomarkers act as reliable indicators of disease severity and recovery, making them useful outcome measures in future dengue treatment trials? 4. How accurately do simplified diagnostic tests identify dengue compared with laboratory reference methods, and are they suitable for use in research and clinical settings in low- and middle-income countries? Participants will be observed without any intervention throughout their hospitalisation. Participants will be be asked to provide informed consent for: * Recording of their routine clinical data * Regular blood tests * Regular ultrasound scans * A follow up appointment.