Pneumocystis carinii cyst. Coloured transmission electron mMicrograph of the resistant cyst stage in the life cycle of P. carinii, the single- celled protozoan parasite that causes pneumonia in victims of AIDS (Acquired Immune Deficiency Syndrome). The cyst is bound by a triple-layered cell wall and contains three insnuclear bodiesins (blue, yellow in lower half). These nuclear bodies in P. carinii cysts develop into free- living organisms called trophozoites, which infect lung tissue and cause pneumonia. AIDS- related pneumonia is difficult to treat and often fatal. Magnification: x1,500 at 6x7cm size. x5000 at 10x8ins