This chapter identifies animal diseases that are host specific and reviews the effects of various human diseases on animals. It also identifies history, etiologic, human Infection, diagnosis, public health aspects and therapeutic aspects of each disease important to epidemiology and prevention. In the developed countries possessing diagnostic capabilities to allow diagnosis of chlamydiosis, the number of people in close contact with the most commonly infected farm animals is relatively low and chlamydiosis is seldom diagnosed. Chlamydia trachomatis, the most common cause of human chlamydiosis, is a human pathogen which is not considered to cause zoonotic infections since it has no known animal reservoir. The mammalian chlamydioses are contagious diseases, and although arthropods have been shown to be infected with chlamydia they are believed to play inconsequential, if any, roles in their transmission. Early diagnosis and initiation of antibiotic treatment are important for success in the chemotherapy of human and other animal chlamydioses.