Pneumonias in the compromised host

The approach to the compromised host with fever and pneumonia depends on an analysis of the patient's immune defects, duration and degree of immunosuppression, the acuteness or subacuteness of the pulmonary process and the appearance and behavior of the infiltrates seen on the chest radiograph or CT/MRI scan. Bacterial pneumonias causing community-acquired pneumonia (CAP) in compromised hosts are caused by the same pathogen group that affects normal patients with CAP, e.g., S. pneumoniae, H. influenzae, or Legionella. Because the bacterial pathogens causing CAP in compromised hosts are predictable, empiric treatment for CAP in the compromised host is the same as in the normal host. Compromised hosts with nonbacterial pneumonias may be caused by a wide variety of pathogenic microorganisms, which precludes empiric therapy in these patients. Nonbacterial pathogens causing invasive disease in compromised hosts require a tissue diagnosis. Transbronchial biopsy or open-lung biopsy are the preferred diagnostic modalities. Specific therapy is based upon demonstrating a specific pathogen invading lung parenchyma. Clinicians should be aware of the many noninfectious mimics of pneumonia in compromised hosts that, if diagnosed, may be treatable.