The Fever Clinic Acute Respiratory Cohort

Inclusion Criteria: * Age ≥ 18 years (based on official identification document). * Presence of fever at the time of visit or within 24 hours prior to the visit, defined as a documented temperature of ≥37.3°C axillary/tympanic, ≥37.8°C orally, or ≥38.0°C rectally. * The primary cause of fever, as determined by the attending physician, is a respiratory tract infection. * Blood draw is required as part of standard clinical care. * Ability to comprehend the study's purpose, procedures, and potential risks, and willingness to provide written informed consent. Exclusion Criteria: * Inability to provide independent informed consent due to impaired consciousness or other reasons. * Inability to comply with the study procedures. * Medical contraindications to blood sampling, such as severe coagulopathy (e.g., INR \> 3.0, platelet count \< 50 × 10⁹/L). * Diagnosed with a terminal illness (e.g., advanced malignancy, severe multiple organ failure) and a life expectancy of less than 3 months.