A prion is a nonliving, infectious agent made of misfolded protein. It can reproduce by making normal, healthy proteins misfold themselves with the aid of its host's biological machinery, like a virus. "Prion" is short for "proteinaceous infectious particle."
The idea of an infectious agent lacking nucleic acids for reproduction was initially met with skepticism when the idea was proposed in the 60s, because such a phenomenon had never been observed before. While nucleic acid-based entities are well-understood, the mechanism of reproduction among prions is still not entirely clear. Prions found in animals exclusively infect the brain, are fatal and untreatable. Prions have been discovered in fungi, but rather than damaging them, it is theorized that they may play a symbiotic role in the fungi's immune system.
Prions are responsible for the outbreak of Mad Cow Disease in Britain during the early 00s, which led to the destruction of thousands of livestock, and international fear about the possibility of the disease spreading from bovines to humans. The prion protein was not isolated until 1982, when Stanley B. Prusiner discovered it and coined the term. He was awarded the Nobel Prize in physiology or medicine in 1997 for his discovery.
One of the first instances of prion-based disease to be investigated was that of scrapie, a disease occurring among sheep and goats. This disease was known since the 18th century, but it was not until the 60s that scientists began to suspect that scrapie was not a typical disease, reproducing outside of the usual means of nucleic acids. Scrapie, like most prion-based diseases, is fatal to the animal it infects, causing horrible symptoms before death. The name of the disease comes from the disturbing behavior of infected sheep, to scrape off their fleece against rocks. The prion also causes an inability to walk, convulsive collapse, and excessive lip-smacking.
Prions are responsible for diseases included in the class known as transmissible spongiform encephalopathy. (The scientific name for Mad Cow disease is bovine spongiform encephalopathy.) The disease is so named because it causes the brain of the victim to degenerate into a sponge-like consistency.
Most prions are not transmissible between species, but cases of humans contracting a version of the disease called vCJD has been reported, with 4 deaths occurring in the United Kingdom town of Armthorpe. Reports such as these led to the worldwide terror accompanying Mad Cow disease. Since the outbreak, several dozen instances of prion infection have been reported in a variety of countries around the world, such as Japan, but many big livestock-producing countries remain disease-free. The meat industry in the United States suffered terribly when news of the disease hit American shores, although the probability of a bovine prion infecting a human is incredibly small.