Transmissible Spongiform Encephalopathies (TSEs) are severe degenerative diseases which affect a wide range of mammals - they include bovine spongiform encephalopathy (BSE or “Mad Cow Disease”), fatal familial insomnia, Creutzfeldt-Jakob disease, kuru, scrapie, and feline spongiform encephalopathy.
All have been found to be 'associated' with prions (misfolded proteins). However, no direct proof of prions as the causative agent has yet been found in any of the diseases studied.
“Several scientific observations remain unexplained by the prion hypothesis: It is known that mice with severe combined immunodeficiency do not develop scrapie following inoculation with brain tissue from animals infected with scrapie, suggesting that either the role of immunity in prion pathogenesis is incompletely understood or that there is some other flaw in current understanding of prion pathophysiology. More recently, it has been shown that scrapie and Creutzfeldt–Jakob disease may require agent-specific nucleic acids for transmission of infection. For these reasons, the prion/TSE hypothesis incompletely accounts for the observed data”
Source : Wikpedia
In addition, it has not yet been established what causes a normal protein to misfold.