Severe<unk> can be diagnosed with either one major criterion or three or more minor criteria.
Respiratory rate > 30 breaths/min
PaO2/FiO2 ratio < 250
Multilobar infiltrates
Confusion/disorientation
Uremia (blood urea nitrogen level > 20 mg/dl)
Leukopenia (WBC < 4,000 cells/µL)
Thrombocytopenia (platelet count 100,000 cells/µL)
Hypothermia (<36.8C)
Hypotension requiring aggressive fluid resuscitation
Septic shock with need for vasopressors
Respiratory failure requiring mechanical ventilation
Leukopenia due to infection alone (ie, not due to chemotherapy)
Pneumonia 
Pulmonary
Pleuritis
Airway obstruction
Atypical pneumonia
Anaphylaxis
Lung abscess
Angioedema