Prions are a very unusual form of infectious agent, consisting of a misfolded version of an otherwise harmless brain protein called prion related protein (PrP). Remarkably, prions are able to turn molecules of normal PrP into more prions, which leads to a chain reaction causing the accumulation of more and more prions. Prions then aggregate into clumps or plaques [plahks] in the brain, causing surrounding brain cells to die, so the brain develops a spongy appearance. This, in turn, causes muscle wasting and loss of muscle control. This resulting inability to coordinate breathing ultimately leads to death.
Some cases of prion disease are inherited (passed on from parents to their offspring), but prions are also considered to be infectious agents because they can be transmitted between humans, or between humans and animals. People can become infected by eating the brains or nervous tissue of animals or other people with prion disease.
The best known examples of prion diseases are BSE (bovine spongiform encephalopathy, popularly known as ‘mad cow disease’) in cattle, and vCJD (variant Creutzfeldt–Jakob disease) in humans. BSE is caused by cattle-to-cattle transmission, vCJD is caused by humans eating products derived from cattle with BSE. The transmission of prions between humans is of course very rare, but has been documented in certain Papua New Guinean tribes that practice funerary cannibalism – the eating of dead family members – where it causes a disease known as Kuru. Understandably, funerary cannibalism is now strongly discouraged.