Transmission Electron Micrograph (TEM) of a section of human lung alveolus containing Pneumocystis jirovecii (formerly known as P. carinii), mainly trophozoites and a collapsed empty cystic stage. Pneumocystis jirovecii is a yeast-like fungus; pneumonia caused by P. jirovecii is one of several opportunistic infections afflicting patients with compromised immune systems. Magnification 4000x, 6.5" x 6.5" image size.