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 The number 1 replicate by mitosis The number 2 to The number 3. Sexual phase: haploid trophic forms conjugate The number 1 and produce a zygote or sporocyte (early cyst) The number 2. The zygote undergoes meiosis and subsequent mitosis to produce eight haploid nuclei (late phase cyst) The number 3. 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 The number 4. A trophic stage, where the organisms probably multiply by binary fission is also recognized to exist. The organism causes disease in immunosuppressed individuals.