In an effort to discern the common features of the transmissible slow diseases, certain aspects of the transmissible subacute spongiform encephalopathies and of visna, maedi and Aleutian disease are discussed. Epidemiological studies indicate a genetic predisposition to these diseases in the natural host species. The causal agent is widely disseminated throughout the body during the long latent period but the progression of the disease process is not inhibited by an immunological response by the host. In each disease the clinical phase, which progresses slowly but inevitably towards a fatal termination, is associated with the proliferation of a specific cell type. The pathogenesis and pathology of a disease is determined by the type of proliferating cell.