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.

Inclusion Criteria:

• Age ≥18 years old
• Admitted to hospital (or in an emergency department with anticipated hospital admission) for the management of a suspected or confirmed acute infectious disease.
• Onset of symptoms of an infectious disease within the past 30 days.
• Informed consent for study participation by the participant or a surrogate decision maker if the participant lacks capacity for consent.

Exclusion Criteria:

• Current imprisonment (this does not include quarantine for an infectious disease).
• Patient undergoing comfort care measures only such that treatment focuses on end- of-life symptom management over prolongation of life.
• Expected inability or unwillingness to participate in study procedures.
• In the opinion of the investigator, participation in the study is not in the best interest of the patient.

Participants will be enrolled at study sites globally. Study personnel at the enrolling site will confirm eligibility and obtain consent prior to beginning study procedures. Signing of the informed consent document signifies study entry and Study Day 0. The study population includes adult patients admitted to a hospital with a suspected or confirmed acute infection. All participants will meet the eligibility criteria below. Specific studies executed with this master protocol may narrow the eligibility criteria to a more specific population (that is, a subset of patients who meet the eligibility criteria in this overarching protocol).