Prion disease
A prion disease (also called transmissible spongiform encephalopathy) is a disease which is caused by prions. Prions are structurally altered versions of small proteins that are normally expressed in cells.  Unlike diseases that are caused by gene mutations resulting in the expression of a mutant protein, prions are able to replicate and transmit diseases through physical contact with normal proteins. This causes a protein's structure to change from the normal state to the prion state.