prions are protein rods that are the cause of a class of diseases called subacute spongiform encephalopathies.
prions induce existing endogenous prion protein (PrPC) in the host organism to take on extremely stable, rogue mis-folded forms (PrPSc) that is responsible for the formation of amyloid plaques and neurodegeneration.
PrPC may have a role in myelin repair, long term memory, the sense of smell, and in the ability of stem cells to regenerate bone marrow.
all known prion diseases affect the structure of the brain or other neural tissue, and all are currently untreatable and are always fatal.
prion diseases tend to have long incubation periods measured in years or decades and tend to be twice as long when the disease crosses to another species.
most are spread primarily by eating affected neural tissue, but variant CJD also affects lymph tissues and blood, and thus can be transmitted by blood transfusions or organ transplants.
fungal prions appear to allow another mechanism of inheritance and provide fungi with evolutionary advantage.
originally discovered in the 1960's when a tribe of cannibals in New Guinea were found to be dying of kuru in 1957, a rare form of spongiform encephalopathy, which was eventually traced back to a single native in 1910 who presumably acquired it spontaneously.