Chlamydophila psittaci (now reclassified as Chlamydia psittaci) is an obligate intracellular bacterial pathogen in the family Chlamydiaceae, phylum Chlamydiae. It is the causative agent of psittacosis (also called parrot fever or ornithosis) — a zoonotic infectious disease that can be transmitted from birds to humans. The bacterium has an unusual biphasic life cycle, alternating between the infectious elementary body (EB) form, which can survive outside cells, and the reticulate body (RB) form that replicates inside host cell vacuoles called inclusions. C. psittaci naturally infects a wide range of bird species — most notably parrots and parakeets (psittacines), but also pigeons, poultry, and wild birds — in which it may cause respiratory illness, conjunctivitis, and general systemic disease, or remain latent. Infected birds shed the bacterium in faeces, nasal secretions, and feather dust; humans become infected through inhalation of contaminated aerosols and may develop pneumonia, fever, headache, and systemic illness. As a bacterial pathogen rather than a wildlife species, it is not evaluated under IUCN criteria. Understanding and monitoring this pathogen is important for public health, veterinary medicine, and the welfare of captive and wild bird populations worldwide.