Several naturally occurring viral infections of domestic animals are excellent models for studying unusual host-virus interactions that result in slowly evolving diseases. Two of these are scrapie of sheep and goats and Aleutian disease of mink. The main concern of this project is to characterize these diseases by making observations on their clinicopathologic, virologic, and immunologic features. Such information will help define the mechanisms that give rise to protracted infectious disease not only in animals but also in man.