Causes Pneumocystis pneumonia (PCP), a diffuse interstitial pneumonia.- Yeast-like fungus.- Most infections are asymptomatic.- Immunosuppression (eg, AIDS) predisposes to disease.

- Diffuse, bilateral ground-glass opacities on CXR/CT, with pneumatoceles. - Diagnosed by lung biopsy or lavage.- Disc-shaped yeast seen on methanamine silver stain of lung tissue.

- Treatment: TMP-SMX, pentamidine, dapsone (prophylaxis only), atovaquone.- Start prophylaxis when CD4+ count drops to <200 cells/mm³ in HIV patients.