A prion is a little-understood type of abnormal protein molecule capable of spreading and damaging animal bodies, including humans, in a manner similar to a virus. The most well-known prion disease to date is mad cow disease, which in humans causes Creutzfeldt-Jakob disease.
Prion stands for "proteinaceous infectious particle." They were first theorized to exist in the 1960s, as the hypothetical cause of a number of otherwise unexplainable spongiform encephalopathies - deadly progressive illnesses which cause holes to appear in the brain, eventually hollowing out the tissue of the central nervous system until it resembles a sponge. American researchers eventually isolated the first prions in the 1980s, and coined the term for them.
Prions are still little understood, but we do know that they can accumulate in the brain until they begin to disrupt the functioning of normal tissue, leading to the appearance of sponge-like holes and gaps. This leads to extremely painful neurological symptoms, including convulsions and dementia. There is currently no known cure for any prion-related disease. Fortunately, outside of certain specific contexts, they are currently very rare diseases. New research may hold out the possibility of targeted therapies including bioengineered anti-prion antibodies, or molecules capable of binding with the prion proteins and rendering them harmless. So far, however, all of this is highly experimental.
The most important and well-known prion-related disease today is mad cow disease (formally known as bovine spongiform encephalopathy, which causes Creutzfeldt-Jakob disease in humans. Mad-cow disease involves prions which have been passed between cattle by eating feed containing (or contaminated with) brain matter from infected animals, and can then be passed on to humans who eat the similarly contaminated meat of infected cattles. There was an outbreak of this disease in British cattle in the 1980s, after which several hundred people died of the disease, as well. Currently Creutzfeldt-Jakob disease, the human form, is extremely rare, striking about one in one million people each year. The disease is always fatal, usually within a year of the appearance of the first symptoms.
There are a number of other prion diseases as well, however. One is kuru, also known as laughing sickness, which once spread among the Fore people of Papua New Guinea as a result of cannibalistic funeral rituals. Like mad cow disease, once symptoms of kuru appeared, the disease was invariably fatal, but it has now been largely eliminated, since cannibalism was outlawed midway through the 20th century. In 2010, Case Western researchers also published evidence of a new prion protein disease in the journal Annals of Neurology.