Light micrograph of Chlamydia psittaci bacteria. This image shows the inclusion bodies of Chlamydia psittaci in host cells after staining with Giemsa stain. The inclusion body is darker than the cytoplasm of the cell but lighter than the cell nucleus, which appears black/purple. The inclusions contain the infective particles of Chlamydia, the elementary bodies. Chlamydia psittaci is an endemic pathogen of all bird species. Humans acquire the disease after inhalation of aerosols when in close contact with birds such as parrots or parakeets. In humans C. psittaci causes psittacosis leading to pneumonia. Supplied by Geoffrey Ridgway, courtesy of Wellcome Images.