A Prospective Observational Cohort Study Protocol for Disease Severity Stratification and Prognosis Assessment in Patients With Pneumonia

1. Inclusion and Exclusion Criteria for Pneumonia Patients 1.1 Inclusion Criteria 1) Age ≥ 18 years old; 2) Meeting the diagnostic criteria for pneumonia (acute onset + new infiltrates on imaging); 3) Time from onset to admission ≤ 72 hours; 4) Signed written informed consent. 1.1.1 Inclusion Criteria for Severe Pneumonia Patients A diagnosis of "severe pneumonia" can be made if one of the main criteria or ≥ 3 of the secondary criteria are met. Main Criteria: ① Requires mechanical ventilation with tracheal intubation ② Requires vasopressor drugs after aggressive fluid resuscitation for septic shock Secondary Criteria: * Respiratory rate ≥ 30 breaths/min * PaO₂/FiO₂ ≤ 250 mm Hg * Multiple lobe infiltrates * Confusion and/or disorientation ⑤ Blood urea nitrogen ≥ 20 mg/dL ⑥ Leukopenia (WBC \< 4×10⁹/L) ⑦ Thrombocytopenia (PLT \< 100×10⁹/L) ⑧ Decreased body temperature (central body temperature \< 36 ℃) ⑨ Hypotension requiring fluid resuscitation 1.2 Exclusion Criteria 1. End-stage diseases (such as expected survival \< 48 hours); 2. Long-term immunosuppressive therapy (prednisone \> 20mg/d for more than 3 weeks, or neutropenia after chemotherapy); 3. Pregnant or lactating women. 2. Inclusion and Exclusion Criteria for Patients with Suspected Infection but Negative Pathogen Testing 2.1 Inclusion Criteria <!-- --> 1. Age ≥ 18 years old; 2. Having infection-related symptoms or signs (such as fever, cough, sputum production, elevated white blood cells, etc.), clinically suspected of pneumonia; 3. All pathogen tests are negative within 3 days after admission; 4. Time from onset to admission ≤ 72 hours; 5. Signed written informed consent. 2.2 Exclusion Criteria <!-- --> 1. End-stage diseases (such as expected survival \< 48 hours); 2. Long-term immunosuppressive therapy; 3. Pregnant or lactating women;