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About the image

A slice of a cultured human lung fibroblast is negative-stained and visualized in a transmission electron micrograph. The darker rods and circles visible inside the cell are the Legionella pneumophila bacterium, which causes Legionnaires' Disease. Circles are bacteria in cross-section, while rods show a lengthwise slice. Legionella is an intracellular parasite which causes pneumonia; it is transmitted through water vapors, often by heating or cooling systems. More information is available at the CDC.