Transmissible spongiform encephalopathies are a unique group of brain diseases of different animal species and man that can be transmitted between individuals by inoculation or ingestion of diseased nervous system tissues. The causative agents induce non inflammatory spongiform degeneration of the central nervous system which leads to a progressive loss of brain function including ataxia and paralysis resulting always in death. The agent might consist only of a conformationally altered cellular protein of the host species devoid of nucleic acid. The natural occurrence of transmissible spongiform encephalopathies is restricted to a few species. Thus, under certain conditions, i. e. the feeding of infectious bone meal or scientific experiments, the agents can be transmitted to other species causing lethal disease.