Causal Agent:
Pneumocystis jiroveci (previously classified as Pneumocystis carinii) was previously classified as a protozoa.  Currently, it is considered a fungus based on nucleic acid and biochemical analysis.

Reference:

1. Frenkel JK. Pneumocystis pneumonia, an immunodeficiency-dependent disease (IDD): a critical historical overview. J Eukaryot Microbiol 1999;46:89S–92S.
2. Stringer JR, Beard CB, Miller RF, Wakefield, AE. A New Name (Pneumocystis jiroveci) for Pneumocystis from Humans. Emerg Infect Dis 2002;8:891-896.

Life Cycle:

Pneumocystis stages were reproduced from a drawing by Dr. John J. Ruffolo, South Dakota State University, USA published in Cushion M. Pneumocystis carinii. In: Collier L, Balows A, Sussman M, editors. Topley and Wilson's Microbiology and Microbial Infections: Volume 4 Medical Mycology, 9th ed. New York: Arnold Publishing; 1998. p. 674.  Copyright held by Arnold Publishing and reproduced here by permission of Arnold and Dr. Ruffolo.
Our thanks to Dr. Melanie T. Cushion for her comments on the life cycle text.

References:

1. Ruffolo JJ. Pneumocystis carinii Cell Structure. In: Walzer, PD, editor. Pneumocystis carinii Pneumonia. 2nd ed. Marcel Dekker; 1994. p. 25-43.
2. Cushion MT, Ruffolo JJ, Walzer PD. Analysis of the developmental stages of Pneumocystis carinii in vitro. Lab Invest 1988;58:324-331.

This is a generalized life cycle proposed by John J. Ruffolo, Ph.D. (Cushion, MT, 1988) for the various species of Pneumocystis.  These fungi are found in the lungs of mammals where they reside without causing overt infection until the host's immune system becomes debilitated.  Then, an oftentimes lethal pneumonia can result.  Asexual phase: trophic forms replicate by mitosis to .  Sexual phase: haploid trophic forms conjugate and produce a zygote or sporocyte (early cyst) .  The zygote undergoes meiosis and subsequent mitosis to produce eight haploid nuclei (late phase cyst) .  Spores exhibit different shapes (such as, spherical and elongated forms).   It is postulated that elongation of the spores precedes release from the spore case.  It is believed that the release occurs through a rent in the cell wall.  After release, the empty spore case usually collapses, but retains some residual cytoplasm .  A trophic stage, where the organisms probably multiply by binary fission is also recognized to exist.  The organism causes disease in immunosuppressed individuals.

Geographic Distribution:
Worldwide, in humans and animals.  Serologic evidence indicates that most healthy children have been exposed by age 3 to 4.  Pneumocystis pneumonia (PCP) occurs in immunosuppressed individuals and in premature, malnourished infants.