Prospective, longitudinal studies of people with acute infections are essential to understand risk factors, clinical manifestations, pathobiology, and management strategies. Observational studies can provide data necessary to select interventions and strategies for testing in clinical trials and to develop key design features of trials. Observational studies can be particularly important for establishing an early knowledge base after emergence of a new pathogen, as illustrated by the recent emergence of influenza A (H1N1), SARS-CoV-2, and Mpox. This observational study protocol describes collection of data and biospecimens from sites across the world for characterizing acute infections in hospitalized patients. The protocol is designed to study respiratory infections, infections outside the respiratory tract, established infectious diseases, and emerging infectious diseases. Data generated in this study will be used to efficiently characterize acute infectious diseases and plan future clinical trials.