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There aren't many diseases that are scarier than Creuzfeldt-Jakob disease. Like another disease that is spread among brain-eating cannibals in New Guinea called kuru, Creuzfeldt-Jakob disease causes progressive loss of brain functions that slowly destroy mobility and personality and inevitably result in death over a period of 6 to 24 months.
It's likely that early observations of conditions like kuru and Creuzfeldt-Jakob disease gave rise to the legends of zombies. Like the fictional zombie-ism, these prion diseases can be spread from person to person by eating brains. For the real life diseases, there is no known cure, and the causative agent is known to be a prion.
What's a Prion?
A prion is a protein. Smaller even than a virus, prions are essentially indestructible. They are undetectible by the immune system. Drugs have no effect on them. They can't be irradiated. They can't be dissolved. They exist indefinitely anywhere they land, for instance, on a surgical tool, or a cannibal's fork, on metal surfaces, in the ground, or in the air. It's impossible to clean them off surgical instruments.
Fortunately, prions essentially never appear spontaneously. Either a genetic mutation programs the brain to produce prions, or a prion introduced into the body (typically in a surgical accident, or by cannibalism of an infected person's brain) attracts circulating amino acids and creates "dimers" to make multiple copies of itself. Literally trillions, quadrillions, and quintillions of prions may emerge after infection with a single particle.
In the brain, infectious prions interact with long strands of brain tissue known as fibrils to cause mats of dysfunctional, disruptive proteins known as tangles. These tangles stop the transmission of signals from neuron to neuron in the brain and eventually cause complete disruption of the brain and death.
Prion Diseases Are Deadly
The health conditions caused by prions are untreatable and death is inevitable. There is no known cure for any of the prion diseases of humans or any other animal. Prions transmitted from cow brains to humans cause Creuzfeldt-Jakob or mad-cow disease, prions transmitted from human brains to humans cause the disease of cannibals known as kuru, and there are also prion diseases of deer (which can be contracted by hunters who handle or eat them), ostriches, elk, sheep, goats, moose, mice, cats, nyalas, oryx, and greater kudus.
There are also similarly deadly genetic prion diseases (diseases in which the brain makes its own disease-causing prions) in people such as fatal familial insomnia and a brain-wasting Gerstmann–Sträussler–Scheinker syndrome. But some of the latest research suggests that prions aren't always bad.