During recent decades, it has been firmly documented that chlamydiae are com mon and important pathogens in humans and animals. In humans, chlamydiae are known to cause trachoma (which is still one of the major blinding diseases in the world) and are also one of the most common etiological agents of sexually transmitted diseases and the sequelae thereof, such as infertility. In the last few years, it has also become evident that chlamydiae, i.e., the so-called TWAR agents, are common respiratory tract pathogens. Chlamydiae are also important pathogens in birds and lower mammals, in whom they...