Chlamydiosis are common and hazardous diseases, yet their clinical and morphological aspects are rarely described in the literature. Visceral and brain lesions remain virtually unknown. This book presents the results of long-term studies conducted by the authors and their collaborators, including pathologists, infectologists, neurologists, ophthalmologists, and urologists. It compares clinical (adults, infants, placentas) and experimental materials with modern literary data. Many aspects of this problem have never been addressed in world literature. The text includes well-illustrated clinical, laboratory, and morphological issues of lesions across all locations, with examples from our own practice. It presents modern views on the taxonomy, properties, and diagnostics of Chlamydia. The possible role of the pathogen in the development of non-infectious lesions is also discussed, and directions for further studies is outlined.