[Transmissible spongiform encephalopathies--an unconventional contagious agent].
The clinical appearance, pathogenesis and epidemiology of the most important human and animal spongiform encephalopathies is reviewed. These comprise a heterogenous group of diseases with both transmissible and hereditary elements in their etiology. They are characterized by neurodegenerative changes in complete absence of inflammatory and immunological reaction. After a long incubation time there is a progressive development which usually leads to death. The most usual symptoms are motoric disturbances, coordination problems and mental retardation. The transmissible agent involved is unconventional, probably a modified host protein, called prion. Post-translational alterations of the prion protein seem to be of key importance in the development of pathogenic properties. The mechanism of replication and transmission of prions is not clear.