Transmissible spongiform encephalopathies (TSEs)

Also known as prion (an infectious agent composed of mis-folded proteins in contrast to all other known infectious agents like virus, bacteria, fungus etc.) diseases, are a group of progressive conditions that affect the brain and nervous system of many animals, including humans. Mental and physical abilities deteriorate and myriad tiny holes appear in the cortex causing it to appear like a sponge (hence the term 'spongiform') when brain tissue obtained at autopsy is examined under a microscope.

The disorders cause impairment of brain function, including memory changes, personality changes and problems with movement that worsen over time. (Source - Wikipedia)