They are the products of changes in the frontal lobe of our brains, in the rate of neuron growth, in the shedding of the myelin sheath around our nerve cells.

A team of scientists now says that PrP plays a key role in maintaining myelin, a fatty substance that forms a sheath around nerves and helps transmit nerve signals.

"Because there is no myelin damage at birth, we assumed prions are needed to maintain the quality of the myelin sheath, which diminishes throughout life," says Aguzzi.