Pneumocystis carinii   

• Impression smear, mouse lung, Giemsa-stained
• Cyst appear as “halo”, about 5 micrometer in diameter
• Intracystic bodies are small, 1-2 micrometer, each has reddish nucleus and blue cytoplasm
• Trophozoites can be seen as minute bodies distributed all over the smear
• Recently classified as a fungus