The great interest for transmissible spongiform encephalopathies, transmissible neurodegenerative diseases has been rapidly grown in the last 10 years, especially after their epidemic appearance in domestic and wild animals in Great Britain and the Bovine Spongiform Encephalopathy ("mad cow diseases") in England has provoked the increased interest in veterinary and medical researches. The opinion that prions are due to transmissible spongiform encephalopathies was accepted by Prusiner and al. 1982. Prions are defined as "Little infective pathogens, which contain protein and which are resistant to procedures which modify and hydrolyze nucleic acid". These diseases in animals and humans have a long incubation period and a long clinical course, and are always fatal. Neuropathological features consist of neuronal vacuolization, neuronal death and gliosis with hyperastrocytosis. Transmissible neurodegenerative diseases are transmissible, infectious and inherited diseases. The inflammatory syndrome, immunological reactions or interferon cannot be detected in the central nervous system. The precise diagnosis of transmissible neurodegenerative diseases can be established only by the examination of the central nervous system after biopsy or autopsy. During the last two years a new variation of Creutzfeld-Jakob disease was detected in England which is different from the classical Creutzfeld-Jakob diseases. The clinical course and the absence of EEG features are typical for Creutzfeld-Jakob disease, and neuropathological changes show extensive curu plaques in the cerebrum and the cerebellum. According to the latest researches it has been established that the abnormal PrP, PrPsc are the same or similar strains which cause the bovine spongiform encephalopathy and a new variation of Creutzfeld-Jakob disease, what is of great medical and epidemiological significance.