Prion diseases, or transmissible spongiform encephalopathies (TSEs), occur naturally in several species, including humans, cattle, sheep, and deer, and can be transmitted experimentally to many others. Typically, incubation times are relatively long, extending to 40 years or more in humans; however, after appearance of clinical symptoms, death mostly ensues within less than a year, as a consequence of neurodegeneration accompanied by accumulation of abnormal conformers of the host protein PrP. Natural transmission usually occurs perorally, as exemplified by the kuru epidemic among the Fore people of Papua New Guinea, attributed to cannibalistic practices; the bovine spongiform encephalopathy (BSE) epizootic in the United Kingdom at the end of last century, caused by feeding of contaminated meat-and-bone meal to cattle; or the current epizootic of chronic wasting disease afflicting cervids in 19 states of the United States. Transmission of BSE prions to young humans gave rise to a limited outbreak of a novel illness, variant Creutzfeldt-Jakob disease (vCJD), almost exclusively in the UK. Sporadic cases of prion disease occur at very low frequency in human populations (sCJD) and in cattle herds (atypical BSE), and are attributed to spontaneous generation of prions in the affected individuals. Finally, familial forms of human prion disease are linked to a variety of different, dominant mutations in the PRNP gene, and while afflicted families are rare, penetrance is very high.
Very nice review from an authority in the field.
Prion image courtesy of Russell Kightley Media