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. As early as 1935 the brown dog tick, Rhipicephalus sanguineus, was considered to be a vector of Ehrlichia canis. The importance of this tick in transmitting canine ehrlichiosis has been demonstrated in numerous cases where effective tick control prevented the occurrence and spread of the disease. It is obvious that the emergence of Lyme disease and ehrlichiosis has contributed greatly to an increase in scientific interest in tick-borne zoonoses in the United States and elsewhere. Among various prominent pathological manifestations in fatal canine ehrlichiosis are the extensive invasion of parenchymal organs and perivascular cuffing by plasma cells, particularly of the lungs, meninges, kidneys, and spleen, suggesting an immunopathological etiology.