Coloured scanning electron micrograph of a cluster of Pneumocystis carinii (green) a single- celled parasitic protozoan that causes pneumonia in AIDS patients. This view shows an infected alveolus of the lung in a monkey with simian AIDS. The disease is relatively rare in people with a healthy immune reaction. In people suffering from AIDS, however, in which the immune system is impaired, P. carinii is a major opportunistic infection. Severe episodes of this pneumonia may recur with micro-abscesses forming in the lungs. Antibiotic drugs may help, but pneumocystis pneumonia is a major cause of death in people with AIDS. Magnification: x720 at 6x4.5cm size.