Table 4.

Etiologic agents of pneumonia in immunocompromised hosts

Organism	Comment
Pneumocystis jaroveci	Previously called Pneumocystis carinii; associated with cellular immune defects, including HIV infection; typically seen when CD4 count is less than 200 cells/mm3 or in infants from 3 – 6 months of age
Cryptococcus neoformans	Yeast; intrinsically resistant to caspofungin
Candida spp	May be part of disseminated deep-organ infection
Aspergillus spp	Common cause of nodular lung infection
Zygomycetes	Family of fungi that includes Rhizopus, Mucor, and others; may be resistant to amphotericin B
Nocardia spp	Environmental bacteria; commonly cause infection of lungs, brain, or skin; require long-term therapy
Cytomegalovirus	Pneumonia as part of disseminated disease
Herpes simplex virus and varicella-zoster virus	Pneumonia as part of disseminated disease
Encapsulated bacteria (S. pneumoniae, H. influenzae, Salmonella spp)	Respiratory infections in asplenic hosts or hosts with humoral immune defects
Nosocomial bacteria, including Pseudomonas or enteric gram-negative rods	Consider as cause of pneumonia in neutropenic patients; may be seen in association with central venous catheter infections