Chlamydia psittaci bacteria. Computer illustration showing two life stages of Chlamydia: elementary bodies (extracellular non-multiplying infectious stage, small orange spheres outside the cell) and an inclusion composed of a group of chlamydia reticulate bodies (intracellular multiplying stage, small orange spheres inside the cel) near the nucleus (violet) of a cell. Chlamydia species are atypical bacteria in that they are obligate intracellular parasites, living and reproducing only inside cells. This species causes abortion in animals and lung disease in humans.