Transmission electron micrograph (TEM) of a section of lung tissue from a person suffering from Acquired Immune Deficiency Syndrome (AIDS), showing the presence of the parasitic protozoans, Pneumocystis carinii (arrowed). Pneumonia caused by Pneumocystis carinii is one of several, so- called opportunistic infections that persons infected with the AIDS virus may succumb to. The AIDS virus (an RNA-containing retrovirus) somehow destroys the T4 helper" lymphocytes that are an essential factor in the body's response to infection. Magnification: X 6000 at 6x6 size."