A group of progressive conditions that affect the brain and nervous system of many animals, including humans. According to the most widespread hypothesis, they are transmitted by prions. Mental and physical abilities deteriorate and myriad tiny holes appear in the cerebral cortex, causing it to look like a sponge 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. Prion diseases of humans include creutzfeldt-jakob disease.