The group of persistent viral infections includes latent, chronic and slow infections. The latter can be caused by: conventional viruses (e.g. polyomavirus JC - causative agent of the Progressive Multifocal Leucoencephalopathy (PML)); defective forms of conventional viruses (e.g. defective measles virus, causing Subacute Sclerosing Panencephalitis (SSPE)) and non-conventional infectious agents - prions (proteinaceous infectious particles), causative agents of transmissible spongiform encephalopathies of humans and different animal species. Prion Diseases are unique in that the same pathologic substrate may cause infectious , genetic and sporadic disease. The primary signs of prion diseases are cognitive impairment and ataxia. On histologic analysis of tissue, spongiform degeneration of the brain accompanied by activated astrocytes and microglia is observed. Prion diseases are the least frequent of all persistent infections. However, there is a big scientific interest in these diseases aimed to solve many open questions related to these mysterious diseases. Emergence of the variant Creutzfeldt-Jakob disease in 1996 in Europe manifested a new threat - prions may cross a species barrier, humans may get infection from diseased animals. According to some authors, the list of prion diseases may grow to include several neurodegenerative disorders. The present review discusses current scientific concepts and peculiarities of prion diseases.