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. The etiologic agent of avian chlamydiosis has been separated from the rickettsias and given its own order, Chlamydiales, in the rickettsias and chlamydias. Alternate avian hosts occasionally may become infected by short-term contact with infected natural carriers. The incubation period of naturally occurring avian chlamydiosis is highly variable depending on the numbers of chlamydiae and the pathogenicity of the infecting strain. All chlamydiae have a complex antigenic composition; one antigenic component, a lipoglycoprotein, is immunodominant and elicits the antibodies detectable by commonly used serologic methods. Necropsy of birds suspected of having died of chlamydiosis should be done in a laminar flow safety cabinet whenever possible.