Transmissible Spongiform Encephalopathies (TSEs) are severe degenerative brain 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 which can have pathological effects, and which can also somehow 'transmit' their misfolding tendencies - by an unknown mechanism plugin-autotooltip__plain plugin-autotooltip_bigPrion replication

"Prions are infectious agents composed entirely of a protein material that can fold in multiple, structurally distinct ways, at least one of which is transmissible to other prion proteins, leading to disease in a manner that is epidemiologically comparable to the spread of viral infection. Prions composed of the prion protein (PrP) are believed to be the cause of transmissible spongiform encephalopathies (TSEs) among other diseases."- to normal proteins (ref.). 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 : Wikipedia

* Note : In addition, it has not yet been established what causes a normal protein to misfold.

Also see : Protein foldingplugin-autotooltip__plain plugin-autotooltip_bigProtein structuring

Genes set the order that amino acids (the chemical building blocks of proteins) appear in the proteins which they code for. But, working from the gene, the form which the protein's 3-D structure will take cannot as yet be predicted. The extremely complex shapes in which the protein 'folds' has a profound effect on the properties it has within an organism.