Prions are unique proteins which appear to be responsible for a family of illnesses collectively referred to as Transmissible Spongiform Encephalopathies (TSEs), including scrapie in sheep, kuru in humans, and "mad cow disease" in cows. These prion diseases are all caused by the breakdown of the nervous system, which is readily apparent after autopsy or necropsy. Looking at the tissue of the nervous system, it becomes clear that it has literally been eaten away by these rogue proteins, causing the formation of distinctive holes.
The existence of prions was hypothesized as early as the 1960s, when researchers first formulated the so-called "protein only" hypothesis to explain TSEs. These researchers noted that irradiation of potentially infective tissue did not seem to kill the organism responsible, suggesting that it did not contain nucleic acid, which is susceptible to radiation. The theory met with a great deal of opposition until the 1980s, when researcher Stanley Prusiner coined the term "prion" to describe the highly unusual proteins he had discovered; these proteins did not contain nucleic acid, the backbone of all living organisms on Earth.
The word is taken from the longer term "proteinaceous infectious particle." Despite the fact that prions lack nucleic acid and therefore in theory cannot reproduce, these proteins do, by exhibiting strange folding patterns which subvert otherwise healthy proteins. The protein they're made of is found in many animals, explaining why these diseases strike a wide range of species. Animals and humans can get TSE by ingesting prions in things like infected tissue and through contaminated blood products.
These little proteins are quite stubborn. They resist many techniques used to decontaminate food, raising concerns about the safety of the food supply. Prions cannot be irradiated, and very high temperatures and pressure are required to remove them — temperatures far higher than those consumers would normally use to cook a steak. Fortunately, they appear to lie primarily in nervous system tissue, although consuming meat from an animal with TSE could potentially be dangerous.
These proteins made the news in the 1990s, when numerous cases of Creutzfeldt-Jakob disease (CJD) began to be linked with prions and the consumption of contaminated tissue from cattle and sheep in the United Kingdom. Cases of TSE have clearly been documented in food animals in Europe, and researchers suspect that TSE may also be an issue in the United States as well, perhaps explaining the rising rate of so-called "downer cows," cows which for some reason are unable to stand up for slaughter. Such cattle are not supposed to enter the food supply out of concerns about prion diseases and other conditions which could pose a threat to human health.