Prion diseases have been popularized by extensive media coverage of bovine spongiform encephalopathy (BSE) or "mad cow disease" epidemic, observed in Great Britain since 1986, and new variant Creutzfeldt-Jakob disease (nvCJD), reported for the first time in 1996. In contrast to the classical form of the disease, nvCJD affects younger patients, presents a relatively longer duration of illness and is caused by the same agent as BSE. Evidence from laboratory studies now strongly supports the hypothesis that new variant represents human form of animal disease, linked to exposure, probably through food, to bovine prions. Number of BSE reports in the United Kingdom began to decline in 1993, and has continuously decreased year by year since then, but a great worry spread in European countries in association with new BSE reported cases outside of the Great Britain, and increasing incidence of nvCJD. New epidemiological, clinical, histopathological and experimental data on prion diseases are reviewed, focusing our attention on the possible transmission of prion proteins from animals to humans.